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Monographs  on  Experimental  Biology 

EDITED  BY 

JACQUES  LOEB,  Rockefeller  Institute 

T.  H.  MORGAN,  Columbia  University 

W.  J.  V.  OSTERHOUT,  Harvard  University 


INBREEDING  AND  OUTBREEDING 

THEIR  GENETIC  AND  SOCIOLOGICAL 
SIGNIFICANCE 

BY 
EDWARD  M.  EAST,  Ph.D. 

AND 

DONALD  F.  JONES,  Sc.D. 


MONOGRAPHS  ON  EXPERIMENTAL 

BIOLOGY 

PUBLISHED 

FORCED  MOVEMENTS,  TROPISMS,  AND  ANIMAL 

CONDUCT 

By  JACQUES  LOEB,  Rockefeller  Institute 

THE  ELEMENTARY  NERVOUS  SYSTEM 

By  G.  H.  PARKER.  Harvard  University 

THE  PHYSICAL  BASIS  OF  HEREDITY 

By  T.  H.  MORGAN,  Columbia  University 

INBREEDING  AND  OUTBREEDING:  THEIR  GENETIC 
AND  SOCIOLOGICAL  SIGNIFICANCE 

By  E.  M.  EAST  and  D.  F.  JONES,  Bussey  Institution.  Harvard  Universit 

THE  NATURE  OF  ANIMAL  LIGHT 

By  E.  N.  HARVEY.  Princeton  University 

SMELL,  TASTE  AND  ALLIED  SENSES  IN  THE 

VERTEBRATES 

By  G.  H.  PARKER,  Harvard  University 

BIOLOGY  OF  DEATH 
By  R.  PEARL,  Johns  Hopkins  University 

INJURY,  RECOVERY,  AND  DEATH  IN  RELATION  TO 

CONDUCTIVITY  AND  PERMEABILITY 

By  W.  J.  V.  OSTERHOUT,  Harvard  University 

IN  PREPARATION 

PURE  LINE  INHERITANCE 

By  H.  S.  JENNINGS,  Johns  Hopkins  University 

LOCALIZATION  OF  MORPHOGENETIC  SUBSTANCES 

IN  THE  EGG 

By  E.  G.  CONKLIN,  Princeton  University 

TISSUE  CULTURE 

By  R.  G.  HARRISON.  Yale  University 

THE  EQUILIBRIUM  BETWEEN  ACIDS  AND  BASES  IN 
ORGANISM  AND  ENVIRONMENT 

By  L.  J.  HENDERSON,  Harvard  University 

CHEMICAL  BASIS  OF  GROWTH 

By  T.  B.  ROBERTSON,  University  of  Toronto 

COORDINATION  IN  LOCOMOTION 

By  A.  R.  MOORE,  Rutgers  College 
OTHERS  WILL  FOLLOW 


Monographs  on  Experimental  Biology 

INBREEDING  AND 
OUTBREEDING 

THEIR  GENETIC  AND  SOCIOLOGICAL 

SIGNIFICANCE 

BY     ^^ 

EDWARD  M.  EAST,  Ph.D. 

HARVARD  UNIVERSITY,  BUSSEY  INSTITUTION 

AND 

DONALD  F.  JONES,  Sc.D. 

CONNECTICUT  AGRICULTURAL  EXPERIMENT  STATION 

i6  ILLUSTRATIONS 


PHILADELPHIA  AND  LONDON 
J.  B.  LIPPINCOTT  COMPANY 


i 

t 


COPYRIGHT,    IQIQt   BV  J.    B.   LIPPINCOTT   COMPANY 


Eleclrolyped  and  printed  by  J.  B.  Lippincoti  Company 
The  Washington  Square  Press,  Philadelphia,  U.  S.  A. 


LIBRARY 

N.  C.  State  College 

EDITORS'  ANNOUNCEMENT 

The  rapidly  increasing  specialization  makes  it  im- 
possible for  one  author  to  cover  satisfactorily  the  whole 
field  of  modern  Biology.  This  situation,  which  exists  in 
all  the  sciences,  has  induced  English  authors  to  issue 
series  of  monographs  in  Biochemistry,  Physiology,  and 
Physios.  A  number  of  American  biologists  have  decided 
to  provide  the  same  opportunity  for  the  study  of 
Experimental  Biology.  ' 

Biology,  which  not  long  ago  was  purely  descriptive 
and  speculative,  has  begun  to  adopt  the  methods  of  the 
exact  sciences,  recognizing  that  for  permanent  progress 
not  only  experiments  are  required  but  that  the  experi- 
ments should  be  of  a  quantitative  character.  It  will  be  the 
purpose  of  this  series  of  monographs  to  emphasize  and 
further  as  much  as  possible  this  development  of  Biology. 
Experimental  Biology  and  General  Physiology  are  one 
and  the  same  science,  by  method  as  well  as  by  contents, 
since  both  aim  at  explaining  life  from  the  physico-chemical 
constitution  of  living  matter.  The  series  of  monographs 
on  Experimental  Biology  will  therefore  include  the  field 
of  traditional  General  Physiology. 

Jacques  Loeb, 
T.  H.  Morgan, 
W.  J.  V.  Osteriiout. 

6 


i  ^  ^  Of) 


PREFACE 

It  is  inevitable  that  each  work  planned  as  a  member  of 
a  series  of  biological  monographs  should  be  somewhat 
technical.  Of  necessity  each  must  be  concise.  In  view  of 
the  difficulties  these  limitations  involve,  one  may  hardly 
expect  to  escape  the  criticism  that  the  subject  matter 
often  tends  to  be  esoteric  in  its  nature,  for  few  can  say  in 
Shaw's  odd  fancy,  ^'I  tried  to  do  too  much — and  did  if 
Nevertheless,  there  has  been  a  serious  effort  to  avoid  a 
mere  record  of  the  development  of  a  specific  problem  in 
Genetics  as  an  aid  to  the  general  biologist.  No  one  could 
have  a  professional  interest  in  a  subject  of  this  kind  with- 
out the  desire  that  there  be  some  practical  application 
of  the  results  to  agriculture  and  to  the  many  phases  of 
sociology  where  a  knowledge  of  the  laws  of  heredity  is  a 
first  requisite.  Though  such  applications  of  the  genetic 
conclusions  are  touched  but  lightly  here,  there  is  the  hope 
that  the  non-biological  worker  interested  in  problems  of 
human  welfare  will  find  some  new  thoughts  and  pertinent 
suggestions  in  the  compelling  logic  of  the  controlled 
experiments  described  throughout  the  pages.  At  least 
it  was  with  this  idea  in  mind  that  the  authors  prepared 
the  first  four  chapters.  For  the  zoologist  and  botanist 
the  well-known  facts  and  elementary  principles  there  dis- 
cussed would  have  been  unnecessary. 

7 


8  PREFACE 

The  manuscript  has  been  the  product,  as  it  purports 
to  be,  of  a  very  intimate  collaboration,  and  the  authors 
join  in  acknowledging  their  indebtedness  to  their  fellow 
biologists  for  the  privilege  of  copying  several  illustrations 
as  noted  in  the  legends,  to  Professor  T.  H.  Morgan  for 
helpful  and  suggestive  criticism,  and  to  Mr.  L.  C.  Dunn 
and  Mr.  E.  S.  Anderson  for  assistance  on  the  proofs. 

E.  M.  E. 
D.  F.  J. 

Boston,   September,    1919 


CONTENTS 


CHAPTER  PAGE 

I.  Introduction 13 

II.  Reproduction  Among  Animals  and  Plantb 20 

III.  The  Mechanism  of  Reproduction 36 

IV.  The  Mechanism  of  Heredity 50 

V.  Mathematical  Considerations  of  Inbreeding 80 

VI.  Inbreeding  Experiments  with  Animals  and   Plants 100 

VII.  Hybrid  Vigor  or  Heterosis 141 

VIII.  Conceptions  as  to  the  Cause  op  Hybrid  Vigor 164 

IX.  Sterility  and  Its  Relation  to  Inbreeding  and  Cross-breeding  188 

X.  The  R6le  of  Inbreeding  and  Outbreeding  in  Evolution...   195 

XI.  The  Value  of  Inbreeding  and  Outbreeding  in  Plant   and 

Animal  Improvement 210 

XII.  Inbreeding  and  Outbreeding  in  Man:     Their    Effect  on 

THE  Individual 226 

XIII.  The  Intermingling  of  Races    and  National  Stamina 245 

Literature 266 


ILLUSTRATIONS 

FIG.  PAGE 

1.  Asexual  Reproduction,  an  Amoeba  in  Division 21 

2.  Asexual  Reproduction  by  Means  of  Runners 22 

3.  Hermaphroditism  in  the  Tapeworm  Proglottid 24 

4.  Rhopalura,  an  Example  of  Extreme  Sexual  Dimorphism 26 

6.  Sexual  Reproduction  in  Fucus 26 

6.  Ulothrix,  a  Primitive  Type  of  Sexual  Reproduction 28 

7.  An  Adaptation  for  Self-pollination 30 

8.  An  Adaptation  for  Cross-pollination 34 

9.  Diagram  of  Gametogenesis 38 

10.  Diagram  to  Illustrate  Fertilization 39 

11.  Formation  of  Pollen  Grains  in  the  Lily 40 

12.  Fertilization  in  the  Embryo  Sac  of  the  Lily 41 

13.  Entrance  of  Spermatozoon  through  Membrane  of  Egg  of  Star-fish ...  42 

14.  Diagram  Showing  the  Distribution  of  Sex  Chromosome  in  Protenor . .  43 

15.  Identical  Quadruplets  in  Nine-banded  Armadillo 44 

16.  Diagram  to  Illustrate  Inheritance  of  Sex-hnked  Character 48 

17.  Diagram  Showing  Union  of  like  Gametes 52 

18.  Diagram  to  Illustrate  Mendelism  in  a  Cross  between  Long-Spiked  and 

Short-Spiked  Wheat 54 

19.  Diagram  to  Illustrate  Gamete  Formation  in  a  Dihybrid  in  Indepen- 

dent Inheritance 59 

20.  Diagram  to  Illustrate  Gamete  Formation  in  a  Dihybrid  in  Linked 

Inheritance 63 

21.  Diagram  to  Illustrate  Crossing-over 64 

22.  Curves  Showing  the  Limiting  Values  of  the  Coefficients  of  Inbreeding 

with  Various  Systems  of  Matings 84 

23.  Graphs  Showing  the  Total  Inbreeding  and  Relationship  Curves  for 

the  Jersey  Bull,  King  Melia  Rioter  14th 86 

24.  Graphs  Showing  the  Reduction  of  Heterozygous  Individuals  and  of 

Heterozygous  AUelomorphic  Pairs  in  Successive  Generations  of 

Self-fertihzation 90 

25.  Graphs  Showing  the  Increase  in  the  Body  Weight  with  Age  for  the 

Males  of  Inbred  Albino  Rats 107 

26.  Graphs  Showing  the  Increase  in  Weight  of  Body  with  Age  for  DifTer- 

ent  Series  of  Male  Albino  Rats 108 

11 


12  ILLUSTRATIONS 

27.  Graph  Showing  the  Average  Size  of  Litters  Produced  in  Successive 

Generations  of  Inbreeding  Albino  Rats  by  Brother  and  Sister 
Matings 109 

28.  Goliath,  an  Albino  Rat,  the  Product  of  Six  Generations  of  the  Closest 

Possible  Inbreeding 110 

29.  Representative  Samples  of  Inbred  Strains  of  Maize  after   Eleven 

Generations  of  Self-fertilization 130 

30.  Graphs  Showing  the  Reduction  of  Variability  and  Segregation   of 

Ear  Row  Number  in  Selfed  Strains  of  Maize 132 

31.  Plants  of  Maize  after  Eleven  Generations  of  Self-fertiUzation  and 

Their  Fi  Hybrid 150 

32.  Ears  of  Maize  after  Six  Generations  of  Self-fertihzation  and  Their 

Fi  Hybrid 150 

33.  Graphs  Showing  Growth  Curves  of  Two  Inbred  Strains  of  Maize  and 

Their  First  and  Second  Generation  Hybrids 152 

34.  James  River  Walnut,  Hybrid  Between  Persian  Walnut  and  Butternut  154 

35.  Growth  Curves  of  Parent  Races  and  Fi  and  F2  Hybrids  of  Guinea  Pigs  160 

36.  Diagram  to  Show  How  Factors  Contributed  by  Each  Parent  May 

Enable  the  First  Generation  of  a  Cross  to  Obtain  a  Greater 
Development  than  Either  Parent 175 

37.  Cattaloes,  the  Product  of  Crossing  the  Cow  and  the  Bison 180 

38.  Sterile  Hybrid  Between  Radish  and  Cabbage 192 

39.  Tassels  of  an  Almost  Sterile  Strain  Obtained  by  Inbreeding  Maize . .   196 

40.  Representative  Ears  of  a  Cross  Between  Two  Inbred  Strains  of  Maize  202 

41.  Plants  of  a  Cross  Between  Two  Inbred  Strains  of  Maize 202 

42.  Diagram  Showing  a  Method  of  Double  Crossing  Maize  to  Secure 

Maximum  Yields,  Illustrated  by  Actual  Field  Results 203 

43.  First  Generation  Cross  of  Shropshire  by  Delaine  Merino 212 

44.  First  Generation  Cross  of  Hereford  by  Shorthorn 216 

45.  "Big  Jim"  the  Product  of  a  Pure  Bred  Percheron  Stallion   Mated 

with  a  Grade  Mare  of  the  Same  Breed 220 

46.  First  Generation  Cross  of  Chester  White  and  Poland  China 224 


INBREEDING  AND 
OUTBREEDING 

CHAPTER  I 
INTRODUCTION 

Interest  in  the  effects  of  inbreeding  and  of  outbreed- 
ing is  not  confined  to  the  professional  biologist.  Histori- 
cally these  are  old,  old  problems,  practical  problems  of 
considerable  significance  bound  up  with  man's  gravest 
affairs,  his  marriage  customs  and  his  means  of  subsist- 
ence. In  these  matters,  moreover,  the  passing  of  time 
has  not  diminished  the  value  to  be  attached  to  their  solu- 
tion. The  questions  involved  belong  to  theoretical  biol- 
ogy, it  is  true,  and  the  professional  biologist  may  lay  claim 
to  the  first  satisfactory  analyses ;  but  relatively  his  inter- 
est is  that  of  yesterday,  stimulated  by  the  work  of  Dar\s^in 
in  establishing  the  doctrine  of  Evolution. 

The  intimate  relation  which  the  effects  of  various  sys- 
tems of  mating  bear  to  these  three  subjects  will  be  seen 
more  clearly  from  the  following  brief  explanation. 

Anthropological  investigations  have  shown  that  many 
primitive  peoples  established  rigid  customs  of  exogamy — 
marriage  outside  the  family  or  the  clan.  Such  practices, 
after  their  identification  with  totemic  systems  by  Mac- 
Lennan,  became  the  subject  of  much  notable  speculation. 
In  particular  may  be  mentioned  the  works  of  Frazer,  Lang 
and  Freud.    Yet  these  writers  have  thrown  little  light  on 

13 


14  INBEEEDING  AND  OUTBREEDING 

the  origin  of  outbreeding  as  a  social  habit,  and  have  con- 
tributed nothing  whatever  toward  the  solution  of  the  ques- 
tions of  inbreeding  and  outbreeding  in  the  sense  in  which 
they  will  be  treated  here.  It  is  probable,  indeed,  that  these 
customs  usually  originated  without  regard  to  matters  of 
physical  inheritance.  The  tribes  concerned  had  seldom 
risen  to  a  state  of  culture  where  the  welfare  of  their  de- 
scendants might  be  expected  to  cause  anxiety,  since  in  few 
cases  had  there  been  that  development  of  animal  hus- 
bandry necessary  for  the  first  glimpse  into  the  mysteries 
of  heredity. 

These  observations  do  not  necessarily  apply  to  the 
marriage  folkways  which  developed  in  western  Asia  and 
Europe  and  were  passed  on  to  the  United  States.  Our 
laws  preventing  marriages  between  certain  degrees  of 
kinship  have  been  moulded  by  the  touch  of  various  civil- 
izations, but  in  the  main  they  are  a  legal  heritage  from  the 
code  of  Hammurabi  through  the  Hebraic  Tahnud.  Since 
they  are  based  largely  upon  the  customs  of  pastoral  na- 
tions, it  may  be  they  had  some  foundation  in  experience, 
half-truths  drawn  from  casual  and  fragmentary  observa- 
tions of  the  shepherd  and  the  cattleman.  There  is  no  his- 
torical record  of  such  rational  basis,  however.  Many  of 
the  conventionalisms  rigidly  stabilized  by  the  hand  of  re- 
ligious authority  have  not  the  slightest  biological  justifi- 
cation. Witness  the  English  laws  preventing  marriage 
with  a  deceased  wife's  sister.  On  the  other  hand,  if  there 
had  not  been  a  dim  but  real  fear  of  evil  consequences  aris- 
ing from  inbreeding,  there  would  be  something  extraordi- 
nary in  the  frequencies  with  which  taboos  against 
consanguineous  matings  have  persisted.  Among  the 
peoples  contributing  to  European  civilization,  caste  sys- 


INTRODUCTION  15 

terns  have  been  common,  and  the  logical  outcome  of  a 
caste  system  is  marriage  between  near  relatives.  Tnclo 
of  race  encourages  inbreeding  among  the  ruling  class,  and 
power  within  that  ruling  class  prompts  the  perpetuation 
of  a  serving  class  in  the  same  manner.  Why,  then,  should 
exogamy  have  been  continued  so  commonly  throughout 
epochs  marked  by  rational  thought  and  a  high  degree  of 
culture?  It  is  true,  there  are  exceptions  to  this  general 
rule.  Rather  intense  inbreeding  was  practiced  both  in 
Egypt  and  in  Greece  when  they  were  at  the  height  of  their 
power  and  influence.  Nevertheless,  exogamic  customs 
have  prevailed.  They  exist  in  Europe  and  America  at  the 
present  day,  and  it  is  natural  to  wish  to  know  whether 
there  is  any  biological  justification  for  them. 

Let  us  propose  three  questions  which  will  show  the 
sociological  bearing  of  the  problems  under  consideration. 

1.  Do  marriages  between,  near  relatives,  wholly  by  rea- 
son of  their  consanguinity,  regardless  of  the  inheritance 
received,  affect  the  offspring  adversely? 

2.  Are  consanguineous  marriages  harmful  through  the 
operation  of  the  laws  of  heredity? 

3.  Are  hereditary  differences  in  the  human  race  trans- 
mitted in  such  a  manner  as  to  make  matings  between 
markedly  different  peoples  desirable  or  undesirable, 
either  from  the  standpoint  of  the  civic  worth  of  the  indi- 
vidual or  of  the  stamina  of  the  population  as  a  whole? 

Correct  answers  to  these  questions  are  a  matter  of 
more  importance  than  a  superficial  consideration  indi- 
cates. Settled  in  accordance  with  the  biological  facts, 
they  aid  in  establishing  a  concrete  scientific  basis  for 
marriage,  divorce  and  immigration  laws;  they  give 
grounds  for  predicting  the  changes  to  be  expected  in  the 


16  INBEEEDING  AKD  OUTBREEDINQ 

body  politic  due  to  differential  fecundity,  birth  control, 
and  other  agencies  by  which  the  character  of  the  popula- 
tion is  shifted;  they  even  have  some  relevancy  to  many 
problems  which  one  might  suppose  were  wholly  of  an 
economic  nature,  such  as  minimum  wages  and  mothers' 
pensions. 

The  second  series  of  phenomena  arousing  interest  in 
the  results  of  inbreeding  and  outbreeding  comes  from  ob- 
servation upon  domestic  animals  and  cultivated  plants. 
Plants  are  included  by  courtesy,  though  in  reality  intelli- 
gent plant  breeding  hardly  began  until  the  nineteenth  cen- 
tury, and  the  methods  adopted  were  taken  from  the  pro- 
cedures in  use  by  animal  breeders,  with  such  modifications 
and  improvements  as  the  peculiarities  inherent  in  vegeta- 
tive propagation  made  necessary.  Animal  breeding,  on 
the  other  hand,  is  a  very  ancient  occupation,  and  more  or 
less  accurate  data  on  the  effects  of  interbreeding  near 
relatives  as  compared  with  the  effects  of  crossing  differ- 
ent strains  must  have  been  collected  by  all  of  the  old 
agricultural  peoples.  Since  there  is  no  question  that 
under  certain  circumstances  inbreeding  does  produce  un- 
desirable results — defectives,  dwarf -forms,  sterile  indi- 
viduals, etc. — it  may  be  that  their  experience  was  at  the 
base  of  some  of  the  antagonism  toward  close-mating  in 
the  human  race.  Or,  it  is  possible  that  early  breeders  ob- 
served the  phenomenon,  common  to  both  animals  and 
plants,  that  when  two  unrelated  stocks  are  crossed  the  hy- 
brids thus  produced  are  often  more  vigorous  than  either 
parent — the  phenomenon  of  hybrid  vigor  or  heterosis,  as 
it  is  called  at  the  present  time.  There  is  no  proof  of  such  a 
sequence  of  ideas,  but  it  seems  to  be  a  logical  hypothesis. 
At  any  rate,  the  views  of  the  animal  raisers  regarding 


INTRODUCTION  17 

inbreeding  and  the  traditions  regarding  marriage  of  near 
kin  are  very  similar.  The  great  majority  of  breeders  have 
an  ineradicable  fear  of  evil  consequences  if  their  matings 
are  too  close.  Only  here  and  there  a  few  fearless  ones 
have  used  systems  of  extremely  close  mating  to  perpetu- 
ate their  breeds,  and  by  such  methods  have  built  up  in- 
valuable races  of  horses,  cattle,  swine  and  poultry.  But 
here  a  dilemma  appears.  Inbreeding  has  deplorable  re- 
sults in  certain  cases,  yet  in  other  instances  the  returns 
have  been  gratifying.  What  is  to  be  the  future  practice! 
To  be  more  than  mere  trial  and  error,  it  must  be  founded 
upon  a  cogent  analysis  of  the  whole  subject. 

Finally,  interest  in  the  effect  of  various  systems  of 
mating  as  natural  phenomena  has  been  stimulated  by  the 
study  of  organic  evolution.  The  circumstantial  data  of 
comparative  morphology  show  that  in  nature  problems 
similar  to  those  of  man  have  arisen.  If  these  problems 
are  investigated  some  light  may  be  thrown  upon  his  diffi- 
culties. Sexual  reproduction  has  been  the  most  success- 
ful method  of  providing  for  the  propagation  of  animals 
and  plants.  Does  sexual  reproduction,  therefore,  possess 
an  advantage  over  other  methods?  Would  it  otherwise 
have  persisted  as  it  has  in  both  kingdoms?  It  probably 
was  not  the  original  method  of  reproduction.  Asexual 
reproduction,  reproduction  by  simple  vegetative  division, 
appears  to  have  held  the  stage  when  animals  and  plants 
were  simple  and  unspecialized.  Then,  in  all  probability, 
came  sexual  reproduction  with  separation  of  the  sexes. 
Secondarily,  however,  numerous  species  arose  in  both 
kingdoms  wherein  the  sexes  are  united,  male  and  female 
cells  being  produced  in  the  same  individual.  Thus  a  sys- 
tem of  mating  entailing  the  greatest  possible  amount  of 


18  INBREEDING  AND  OUTBREEDINa 

inbreeding  was  established.  But  this  system  appears  to 
have  been  deticient.  {Some  evolutionaiy  advantage  asso- 
ciated with  separation  of  the  sexes  was  lost.  There  is 
reason  for  assuming  that  this  advantage  was  connected 
with  cross-fertilization,  for  tertiary  developments  in  each 
kingdom  brought  about  numerous  mechanisms  whereby 
cross-fertilization  was  established  in  hermaphroditic  or- 
ganisms. Still,  in  spite  of  the  obvious  success  of  bisexual 
and  of  cross-fertilized  species,  as  shown  by  their  fre- 
quency, numerous  self -fertilized  species,  and  even  species 
reproducing  exclusively  by  asexual  methods  have  kept 
their  places  in  the  struggle  for  existence.  Both  inbreed- 
ing and  outbreeding  systems  have  developed  side-by-side 
under  natural  conditions.  The  data  of  comparative  mor- 
phology, therefore,  seem  as  contradictory  as  those  from 
anthropology  and  agriculture. 

The  puzzles  presented  by  these  general  facts  taken 
from  history,  husbandry  and  biology  have  one  common 
feature.  They  centre  on  the  problem  of  inheritance.  For- 
tunately, though  less  than  two  decades  have  passed  since 
the  application  of  quantitative  experimental  methods  to 
biology  became  somewhat  general,  the  mechanism  of 
heredity  is  no  longer  a  riddle;  and  to-day  the  effect  of 
inbreeding  and  outbreeding  on  plants  and  animals  can 
be  described  in  considerable  detail  and  interpreted  with 
singular  precision.  Having  this  interpretation  it  may  be 
applied  to  the  three  fields  of  interest  we  have  described. 

In  the  ensuing  pages  the  important  controlled  experi- 
ments in  inbreeding  and  outbreeding  necessary  for  an 
orderly  and  consistent  interpretation  of  the  facts  are  dis- 
cussed. Uncontrolled  experiments,  casual  observations 
of  stock  breeders,  data  on  human  marriages  between  near 


INTRODUCTION  19 

relatives,  have  been  omitted  designedly.  Numerous  data 
of  these  types  have  been  available  for  many  years,  but 
they  have  been  of  little  service  in  clarifying  the  situation. 
This  is  not  altogether  due  to  their  fragmentary  character 
in  point  of  time,  or  even  to  the  fact  that  they  usually  lack 
the  precision  necessary  in  data  to  be  used  in  the  analysis 
of  such  complex  phenomena.  Data  for  a  limited  number 
of  generations  are  often  useful,  and  precision  is  a  relative 
matter.  The  truth  is,  the  majority  of  these  records  was 
collected  without  regard  to  the  type  of  fact  required,  and 
without  reduction  to  concrete  numerical  terms.  In  other 
words,  in  records  otherwise  accurate,  critical  data  are 
omitted ;  and  those  given  are  relatively  useless  on  account 
of  their  form. 

A  detailed  application  of  our  conclusions  to  sociology, 
agriculture  and  evolutionary  theory  has  not  been  at- 
tempted. It  is  hoped  that  the  suggestions  along  these 
various  lines  are  sufficient  to  show  how  such  application 
can  be  made;  but  human  direction  of  evolution  either  in 
man  or  in  the  lower  organisms  is  beset  with  difficulties  so 
numerous  and  so  prodigious  that  each  problem  must  have 
its  individual  solution. 


CHAPTER  II 

EEPEODUCTION  AMONG  ANIMALS  AND  PLANTS 

In  order  to  obtain  a  proper  orientation  of  the  problem 
of  inbreeding  and  outbreeding,  one  must  consider  first 
some  of  the  general  facts  regarding  reproduction  among 
animals  and  plants  and  their  relation  to  inheritance. 

The  significant  changes  in  both  kingdoms  have  been 
remarkably  similar.  The  differences  are  differences  in  de- 
tail, and  for  this  reason  they  are  additional  arguments  in 
favor  of  the  idea  that  there  are  special  advantages  asso- 
ciated with  the  coincidences  found  in  the  general  processes 
involved.  For  example,  asexual  propagation  is  more  gen- 
eral in  the  simpler,  sexual  reproduction  in  the  higher  or- 
ganisms. But  sexual  reproduction  in  animals  has  largely 
supplanted  the  asexual  method,  in  plants  sexual  reproduc- 
tion was  merely  added.  Is  this  not  evidence  of  an  im- 
portance to  be  attached  to  the  sexual  method,  apart  from 
a  simple  provision  for  multiplication?  Again,  the  diver- 
sity of  sex  organs  which  has  arisen  among  the  various 
groups  of  animals  and  plants  is  highly  surprising,  yet  this 
dissimilarity  may  be  wholly  of  a  superficial  nature.  When 
examined  solely  with  the  object  of  inquiring  what  systems 
of  mating  these  variations  entail,  the  parallelisms  in  each 
history  stand  out  impressively.  If  these  facts  be  kept  in 
mind  throughout  the  short  discussion  of  heredity  and  re- 
production which  follow,  their  probable  evolutionary  sig- 
nificance is  not  difficult  to  grasp ;  but  if  the  profusion  of 
variation  in  detail,  or  even  the  general  mechanism  of  ac- 
complishing a  particular  result  is  allowed  to  distract 

20 


ANIMAL  AND  PLANT  REPEODUCTION       21 


attention,  the  end  may  be  lost  to  sight  through  admiration 
of  the  ingenuity  of  the  means, 

(^Tiiere  seems  to  be  no  question  but  that  sexual  repro- 
duction is  a  more  recent  means  of  propagation  than  asex- 
ual reproduction.  '  Although  asexual  reproduction  in  the 
narrow  sense,  that  is,  by  means  of  simple  division  or  by 
budding,  is  common  among  the 
protozoa,  the  sponges,  the  coelente- 
rates  and  the  fiat  worms,  it  becomes 
sporadic  in  the  molluscoids  and 
annelids,  and  is  found  in  only 
one  or  two  isolated  instances  in 
forms  as  highly  specialized  as  the 
arthropods  and  the  chordates.  If 
fragmentation  succeeded  by  regen- 
eration of  the  lost  parts  be  conceded 
to  be  a  true  means  of  reproduction, 
however,  echinoderms  and  nematode 
worms  are  included.  Thus  of  all 
the  great  groups  of  animals  only 
certain    worms     (TrocJielminthes) 


FiQ.     1. — Asexual     reproduc- 
tion.     An    amoeba    in    division. 


and  the  molluscs  have  no  asexual  cv.  contractile  vacuole;  ek,  coto- 

,  Bare;    en,    entoaarc;    u,   nurleua. 

reproduction  m  the  usual  sense  or   (Kingsiey  after  sehuize.  cour- 

^  ^  teay  Henry  Holt  &  Co.). 

the  word,  and  zoologists  would 
hardly  feel  safe  in  maintaining  its  absence  in  these  two 
phyla  since  the  life  history  of  so  many  forms  is  unknowai. 
But  since  asexual  reproduction  is  replaced  by  sexual 
reproduction  to  a  greater  and  greater  extent  as  the  higher 
forms  are  reached  one  cannot  avoid  the  conclusion  that 
the  latter  has  proved  to  be  the  really  successful  means 
of  propagation.  Nevertheless,  variations  appeared  in 
highly  specialized  forms  which  permitted  return  to  an 


22  INBEEEDINa  AND  OUTBREEDING 

asexual  type  of  reproduction.  In  the  arthropods,  as  well  as 
in  some  other  forms,  mechanisms  arose  by  which  the  eggs 
developed  without  fertilization.  This  parthenogenetic 
reproduction  has  been  relatively  successful,  but  only  as  a 
stop-gap.  Sexual  reproduction  persists  and  is  used  as  an 
occasional  means  of  propagation.  It  would  seem  that  it 
possessed  advantages  too  great  to  be  given  up  entirely. 
Even  as  sexual  reproduction  is  a  later  method  of 

propagation  than  asexual  reproduc- 
tion, hermaphroditism  appears  to  be 
a  secondary  development  from  forms 
in  which  the  sexes  were  separate 
(gonochorism  or  dioecism).  Omitting 
the  protozoa  in  which  it  is  difficult  to 
decide  such  sexual  differences,  gono- 
chorism is  present  in  every  great 
animal  group  but  the  sponges,  and 
hermaphroditism  everywhere  except 
in  the  Trochelminthes,  although  in 
Nemathelminthes,  Echinodermata  and 
FiQ.  2.— Asexual  repro-  Avthropoda  it  is  rare.    An  extended 

ductioa  by  means  of  runners  .  .  . ,  ^    •       j       a  t  i 

in  the  hawkweed.  (After  experiment  ou  the  subject  or  hermaph- 
roditism certainly  was  made,  but  that 
it  was  an  experiment,  that  hermaphroditism  is  from  the 
evolutionary  standpoint  a  secondary  institution,  is  clear 
if  one  considers  the  anatomical  evidence,  as  is  shown  by 
Caullery.23  Generally,  hermaphroditism  is  a  condition 
associated  either  with  parasitism  or  with  a  sedentary  life. 
Furthermore,  hermaphroditic  organisms  do  not  have  a 
truly  simple  organization.  They  have  a  superficial  simplic- 
ity, due  to  an  adaptation  to  their  mode  of  life,  but  if  one 
compares  hermaphroditic  and  gonochoristic  species  group 


ANIMAL  AND  PLANT  REPRODUCTION       23 

by  group,  for  example  unisexual  land  or  fresh-water 
worms  with  their  bisexual  marine  cousins,  he  finds  the  for- 
mer to  be  the  more  complex,  particularly  as  to  their  sex 
organs.  The  fact  that  the  sponges  are  hermaphroditic 
might  be  considered  as  weighing  against  this  argument, 
but  it  is  not  without  the  bounds  of  probability  that  the 
sponges  are  further  along  in  specialization  than  is  gen- 
erally admitted,  for  to  find  the  substance  nearest  chem- 
ically to  the  so-called  skeleton  of  the  sponges,  one  must 
search  among  the  arthropods — the  product  of  the  spin- 
ning glands  of  certain  spiders  and  insects. 

Hermaphroditism,  pure  and  simple,  however,  was  not 
a  success.  Only  a  few  degenerate  forms  retained  self- 
fertilization  and  persisted.  Among  them  may  be  men- 
tioned the  tapeworms,  certain  crustaceans  {Sacculina) 
parasitic  on  crabs,  and  the  colonial  forms,  bryozoans  and 
tunicates,  the  latter  being  perhaps  the  most  degenerate  of 
all  animals  since  they  are  wholly  unrecognizable  as  rela- 
tives of  the  vertebrates  except  at  one  short  stage  of  their 
life  history.  In  most  of  the  hermaphroditic  types  new 
characteristics  appeared  which  enabled  them  to  exercise 
one  of  the  important  functions  of  bisexuality,  cross-fer- 
tilization, without  giving  up  the  obvious  energy  conserva- 
tion attainable  through  the  production  of  both  sex  cells 
in  a  single  individual. 

In  nearly  all  of  these  forms,  this  was  made  possible  by 
the  development  of  the  eggs  and  of  the  sperm  at  different 
times.  In  a  few  isolated  cases  among  the  turbellarians 
and  the  tunicates  the  eggs  develop  first  and  then  the 
sperm;  the  animal  is  first  a  female  and  later  a  male  (pro- 
togyny).  But  in  a  greater  number  of  species,  the  indi- 
vidual is  first  a  male  and  afterwards  a  female   (pro- 


24 


INBREEDING  AND  OUTBREEDING 


tandry).  In  the  tapeworm  (Fig.  3),  for  example,  each 
segment  contains  a  complete  reproductive  system,  testes, 
ovaries  and  accessory  glands ;  when  young  the  testes  func- 
tion, when  older  the  testes  atrophy  and  the  ovaries  de- 
velop. In  some  of  these  protandrous  species  there  is 
even  a  change  in  the  whole  structure  of  the  body,  includ- 


0000  o. 


J      O  do  o 

0  0    qOOq 


O  °oOO 

o   ooaoo 
,o     " 


'°a.§§ 


^°o?,oO.»5^o?o8.'^ 


O      0  0  n^A»: 
ftOoooO*?? 


Fig.  3. — Hermaphroditism  in  the  tapeworm  proglottid.     K,  genital  pore;  ov,  ovary;  re, 
receptaculum  seminalis;  t,  testes;  u,  uterus;  vd,  vas  deferens.  (Kingsley  after  Sommer). 

ing  the  sexual  orifices.  The  isopods  of  the  family  Cynio- 
thoidcd,  a  group  of  crustaceans  parasitic  on  fish,  furnish 
a  beautiful  illustration.  In  the  male  stage  the  animal  is  a 
typical  crustacean  and  would  be  recognized  as  such  by 
any  layman  with  a  very  slight  knowledge  of  zoology ;  but 
when  the  animal  passes  over  into  the  female  stage  it  be- 
comes merely  a  great  ^gg  sac  many  times  the  previous 
size.    One  would  hardly  suppose  the  two  stages  belonged 


pWPfRTT  UBMRT 
^  C.  State  CoUeg* 


ANIMAL  AND  PLANT  REPRODUCTION   25 

to  the  same  order,  not  to  mention  a  transformation  of  the 
same  individual. 

A  few  other  mechanisms  which  promote  cross-fer- 
tilization have  been  found  in  isolated  cases.  They  are  not 
as  widespread  as  the  one  just  described,  but  are  peculiarly 
interesting  nevertheless.  Among  certain  of  the  cirripedes, 
the  normal  individuals  are  hermaphroditic,  but  in  addition 
a  few  tiny  degenerate  males  are  developed.  They  are  little 
more  than  bags  of  sperm  and  are  calculated  to  make  some- 
what amusing  any  generalization  as  to  the  **  stronger 
sex.''  Darwin,  who  discovered  them,  called  them  com- 
plemental  males.  Another  means  of  preventing  continued 
self-fertilization  is  self-sterility,  a  condition  in  which  self- 
fertilization  is  very  difficult  or  even  impossible  through 
some  physiological  impediment  which  is  not  clearly  under- 
stood. It  was  demonstrated  by  Castle  ^^  for  the  American 
race  of  Ciona  mtestinalis. 

In  what  appear  to  be  the  essential  features,  the 
vicissitudes  of  reproduction  have  been  similar  in  tlie 
vegetable  kingdom.  The  problems  were  solved  in  differ- 
ent ways,  but  the  gross  results  are  largely  the  same.  The 
most  striking  difference  is  the  varied  success  of  certain 
mechanisms.  In  the  animal  kingdom  sexual  reproduction 
wherever  instituted  practically  always  displaced  asexual 
reproduction.  Only  in  a  few  forms  which  are  either  fixed 
or  parasitic  in  their  mode  of  life  did  the  two  methods  per- 
sist side  by  side.  In  plants,  however,  where  the  sessile  is 
the  common  condition,  asexual  and  sexual  reproduction 
have  continued  harmoniously  side  by  side  clear  up 
through  the  angiosperms.  Again,  there  is  a  marked  dif- 
ference in  the  success  of  hermaphroditism.  In  plants 
hermaphroditic  forms  became  the  dominant  types  in  the 


26 


INBREEDING  AND  OUTBREEDING 


highest  and  most  specialized  group,  the  seed  plants,  while 
in  the  highest  group  of  animals,  the  mammals,  only  an 
occasional  individual  showing  rudimentary  hermaphro- 
ditism is  found. 

Just  when  sexual  reproduction  first  originated  in  the 
vegetable  kingdom  is  even  more  of  a  question  than  among 
animals.  Only  a  few  very  simple  types,  the  schizophytes 
(bacteria)  and  myxomycetes,  have  passed  it  by.    Perhaps 


FiQ.  4. 


FiQ.  6. 


\  ^\'    -  T    .  .V'  •  T*"-' '  -^        y^-'  "»>;  I..-  ^•-  -.■■If       I 


-v. 


( 


Fia.  4. — Rhopalura,  an   example   of   extreme  sexual  dimorphism.     (After  Caullery.) 
Fia.  5. — Sexual  reproduction  in  Fucus,  giving  some  idea  of  the  difference  in  size  of 

egg  and  sperms.    Sperms  should  be  about  one-tenth  the  size  shown.     (Bergen  and  Davis 

after  Thuret.) 

it  is  for  this  reason  these  forms  have  remained  the  sub- 
merged tenth  of  the  plant  world.  It  is  tempting,  as 
Coulter  ^2  says,  to  see  the  origin  in  the  Green  Algce, 
There,  in  certain  species,  of  which  Ulothrix  is  an  example 
(Fig.  6),  spores  of  different  sizes  are  produced.  The 
large  ones  having  four  cilia  are  formed  in  pairs  in  each 
mother  cell,  the  smaller  ones  usually  having  two  cilia 
occur  in  groups  of  eight  or  sixteen  in  each  spore-produc- 
ing cell.  Those  largest  in  size  germinate  immediately 
under  favorable  conditions  and  produce  new  individuals. 


ANIMAL  AI^D  PLANT  REPKODUCTION       27 

Those  of  lesser  size  also  germinate  and  produce  new 
individuals,  but  these  are  small  and  their  growth  slow. 
Only  the  smallest  are  incapable  of  carrying  on  their  vege- 
tative functions.  These  come  together  in  pairs  and  fuse. 
Two  individuals  become  one  as  a  prerequisite  to  renewed 
vigor.  Vegetative  spores  become  gametes.  Something 
valuable — speed  of  multiplication — is  given  up  that  some- 
thing more  valuable  in  the  general  scheme  of  evolution 
may  be  attained. 

This  is  indeed  an  alluring  genesis  of  sex.  It  is  rather  a 
genesis  of  sex,  however,  than  the  genesis  of  sex.  Various 
manifestations  of  sex  are  present  in  other  widely  sepa- 
rated groups  of  unicellular  or  simple  filamentous  plants, 
the  Peredinew,  the  Conjugatce  and  the  Diatomece — the 
Conjugated  being  indeed  the  only  great  group  of  plants  in 
which  there  is  no  long  continued  asexual  reproduction. 
In  these  forms  one  cannot  make  out  such  a  good  case  of 
actual  gametic  origin,  but  the  circumstantial  evidence  of 
sex  development  in  parallel  lines  is  witness  of  its  para- 
mount importance. 

After  the  origin  of  sex,  many  changes  in  reproductive 
mechanisms  occurred  in  plants,  but  most  of  them  resulted 
merely  in  better  protection  for  the  gametes,  in  increased 
assurance  of  fertilization,  in  provision  for  better  distri- 
bution, or  in  greater  security  for  the  young  plant. 

First,  perhaps,  there  was  physiological  differentiation 
of  the  gametes.  At  least  such  an  interpretation  may  be 
given  to  the  form  of  conjugation  found  in  Spirogyra  and 
other  Conjugates,  where,  either  by  solution  of  the  wall 
separating  them,  or  by  the  formation  of  a  tube-like  out- 
growth of  one  or  both  cells  so  that  the  ends  touch,  tlie 
contents  of  one  cell  pass  over  to  the  other.     We  may 


28 


INBREEDING  AND  OUTBREEDING 


tliink  of  the   stationary  cell  as  female  and  the  other 
as  male. 

Another  line  of  development,  however,  became  the 
dominant  one  in  the  plant  kingdom  just  as  it  did  in  the 
animal  world.  A  morphological  differentiation  of  the  sex 
cells  occurred.     One  became  a  large  inactive  cell  stored 


FiQ.  6. — Ulothrix,  a  primitive  type  of  sexual  reproduction.  A,  B,  filaments;  C,  «o- 
ospores;  Z),  germination  of  zoospore;  £,  gamete  formation  in  filament;  F,  gametes  and  their 
fusion;  G,  germination  of  zygospore.     (Bergen  and  Davis  after  Dodel.) 

with  food,  the  egg ;  the  other  became  small  and  motile,  the 
sperm.  This  change  is  well  illustrated  in  Fucus  (Fig.  5), 
one  of  the  brown  algae.  It  is  clear  that  such  a  change  in- 
creased the  probability  of  fertilization,  since  many 
sperms  could  be  produced  without  utilizing  a  great  deal 
of  energy,  and  since  the  attraction  of  the  egg  for  the  sperm 
was  presumably  augmented. 

A  further  stage  in  the  evolution  of  sex  was  reached 
when  the  cells  producing  the  eggs  or  the  sperms  were 


ANIMAL  AND  PLANT  REPKODUCTION       29 

differentiated,  thus  providing  for  protection  of  the 
gametes.  Such  organs  of  various  types  and  known 
by  different  names  have  persisted  throughout  all  the 
higher  plants.  One  may  call  them  ovaries  and  spermaries 
and  thus  keep  in  mind  that  in  animals  the  same  types  of 
change  occurred. 

The  final  step  in  the  general  development  of  sexuality 
is  the  restriction  of  the  formation  of  sex  organs  to  a  par- 
ticular phase  in  the  plant's  life,  which  on  this  account  is 
known  as  the  gametophyte.  The  remaining  stages  are 
known  as  non-sexual  or  sporophytic,  because  they  are 
characterized  by  the  production  of  non-sexual  reproduc- 
tive cells,  the  spores.  The  liverworts,  the  mosses,  the 
ferns  and  the  seed  plants  are  thus  set  apart. 

Since  these  two  phases  alternate  with  each  other,  pairs 
of  reproductive  cells  of  the  gametophyte  producing  the 
sporophyte,  and  the  non-sexual  spores  of  the  latter  giving 
rise  to  the  gametophyte,  the  sequence  has  retained  the 
name  of  alternation  of  generations. 

In  the  higher  liverworts  and  mosses  the  gametophyte 
carries  on  the  greater  part  of  the  nutritive  work,  but  in 
the  ferns  the  sporophyte  becomes  the  dominant  structure ; 
while  in  the  seed  plants  the  gametophyte  has  degenerated 
until  it  consists  of  but  two  or  three  cell  divisions. 

There  is  no  question  but  that  all  of  these  numerous 
changes  are  merely  insurance  against  the  future,  some- 
thing that  may  be  said  of  seed  production  as  a  whole,  since 
the  seed  is  but  the  younger  generation  nourished  on  the 
parent  stem.  And  it  is  interesting  to  note  that  just  as 
animals  and  plants  paralleled  each  other  in  cramote  pro- 
tection and  provisions  for  assuring  fertilization,  so  also 
the  final  step  in  each,  the  mammals  and  the  seed  plants, 


30  INBREEDING^  AND  OUTBREEDING 

was  the  protection  of  the  young.  In  certain  particulars, 
however,  the  higher  plants  did  not  simulate  the  higher 
animals  in  their  reproductive  evolution,  and  it  is  not  diffi- 
cult to  see  the  reason  for  the  divergencies.  Plants  re- 
tained asexual  reproduction  as  an  alternative  method  of 
propagation,  and  made  a  success  of  hermaphroditism. 
The  obvious  necessity  for  both  was  their  fixed  condition, 
their  slavery  to  the  soil ;  but  if  hermaphroditism  with  its 
simplest  implication,  self-fertilization,  had  become  domi- 


FiG.  7. — Adaptation  for  aelf-pollination  by  means  of  spiral  twietings  of  stamens  and  style. 

(After  Kerner.) 

nant,  there  would  have  been  little  from  their  life  histories 
upon  which  to  base  an  argument  regarding  the  respective 
virtues  and  defects  of  inbreeding  and  outbreeding.  This, 
however,  was  not  the  case.  Many  plants  characterized  by 
autogamy  persisted  and  flourished.  They  even  developed 
numerous  devices  promoting  self-fertilization  (Fig.  7), 
such  as  pollination  before  the  flower  opens,  inclination 
of  the  anthers  toward  the  pistil  or  the  pistil  toward  the 
anthers,  rapid  elongation  of  the  pistil  through  a  ring  of 
stamens,  or  various  torsions  of  the  accessory  floral  parts ; 
but  it  seems  perfectly  clear  from  the  exhaustive  investi- 
gations on  the  fecundation  of  plants  made  in  recent  years 


ANIMAL  AND  PLANT  REPRODUCTION       31 

that  only  an  extremely  small  percentage  of  the  species  of 
flowering  plants  which  have  held  their  own  to  the  present 
day  in  the  struggle  for  existence,  have  adopted  a  method 
of  fertilization  which  permits  no  crossing.  Some  of  our 
most  vigorous  cultivated  plants — tobacco,  wheat,  peas  and 
beans — are  naturally  and  usually  self -fertilized,  but  they 
each  and  every  one  have  their  flowers  so  arranged  as  to 
pennit  an  occasional  cross. 

At  the  same  time,  one  would  be  too  hasty  if  he  con- 
cluded from  these  facts  that  continuous  self-fertilization 
or  other  means  of  reproduction  which  result  in  a  single 
line  of  descent  is  incompatible  with  inherent  racial  vigor. 
At  least,  there  is  evidence  that  various  species  which  seem 
well  able  to  hold  their  own  seldom  resort  to  crossing  as  a 
means  of  propagation,  yet  one  could  hardly  use  them  as 
examples  of  degeneration.  As  illustrations,  there  is  no 
need  to  go  below  the  flowering  plants,  either,  although  if 
one  desires  an  example  of  a  long-continued  evolution  of 
species  and  genera  without  any  form  of  sexual  reproduc- 
tion he  is  forced  to  look  to  the  Basidiomycetes.  In  this 
large  group  the  fungi  are  not  only  asexual  themselves,  but 
appear  to  have  been  developed  in  a  purely  asexual  manner 
from  asexual  ancestors.  But  in  the  flowering  plants, 
many  of  our  most  useful  types — the  potato,  the  banana, 
hops  and  sugar  cane — seldom  have  recourse  to  sexual  re- 
production. It  is  true  many  agriculturists  insist  that 
these  species  sooner  or  later  degenerate  for  this  very 
reason,  but  they  have  never  been  able  to  bring  forward 
one  atom  of  critical  evidence  to  uphold  their  view.  Vari- 
eties of  potatoes  or  of  sugar  cane  do  indeed  degenerate, 
but  it  is  probably  because  of  disease  which  from  their 
method  of  propagation  is  difficult  to  eradicate,  and  not 


32  INBEEEDINa  AND  OUTBBEEDINa 

because  of  the  method  itself.  Again,  if  one  desires  further 
evidence  of  descent  in  a  single  pure  hereditary  line  con- 
sistent with  high  specialization  and  inherent  vigor 
through  long  periods  of  time,  there  is  the  phenomenon 
of  apomixis  to  be  cited.  Apomixis  is  a  general  term  for 
certain  reproductive  anomalies  in  plants  which  are  really 
a  return  to  vegetative  reproduction.  In  a  broad  way  it  is 
synonymous  with  parthenogenesis  in  animals;  but  par- 
thenogenesis in  animals  includes  only  reproduction  from 
an  unfertilized  egg,  while  apomixis  takes  in  reproduc- 
tion from  certain  cells  which  are  not  eggs.  Some  twenty 
or  thirty  species  of  vascular  plants  have  already  been 
found  to  reproduce  in  this  manner,  and  unquestionably 
the  list  is  very  incomplete.  Examples  from  Polypodiacece, 
Ranunculacece  and  Rosacece  are  not  uncommon,  but  in 
particular  it  is  the  Composited,  the  highest  group  of  flow- 
ering plants,  which  seem  inclined  to  make  this  method  of 
reproduction  a  habit.  Of  course,  one  cannot  insist  that 
such  a  return  to  primitive  reproductive  methods  even  by 
a  more  modem  labor-saving  route  is  wholly  for  the  good 
of  the  species  concerned.  No  one  in  possession  of  all  of 
the  facts  could  maintain  the  change  to  be  progressive,  or 
argue  that  the  species  adopting  it  will  have  a  great  future 
as  future  is  measured  by  the  evolutionist.  This  is  not 
the  contention.  We  merely  cite  the  adoption  of  apomixis 
by  flourishing  genera  of  the  most  specialized  and  highly 
developed  plants  as  examples  of  asexual  reproduction 
over  long  periods  without  visibly  harmful  effects.  We  do 
this  because  we  believe  the  emphasis  put  by  Darwin  and 
his  followers  on  supposed  ill  effects  following  any  type 
of  inbreeding  or  asexual  propagation  was  misplaced. 
Certainly  the  majority,  the  great  majority,  of  the  higher 


ANIMAL  AND  PLANT  REPRODUCTION       33 

plants  returned  to  a  type  of  reproduction  which  held  all 
the  advantages  of  bisexuality  by  evolving  means  for  pro- 
moting cross-fertilization.  But  it  is  the  advantage  of 
cross-fertilization  and  not  the  assumed  disadvantage  of 
self-fertilization  that  should  be  stressed.  The  Knight- 
Darwin  Law,  ^'Nature  abhors  perpetual  self-fertiliza- 
tion/' should  read  Nature  discovered  a  great  advantage 
in  an  occasional  cross- fertilization. 

The  higher  plants  made  a  success  of  hermaphroditism 
because  there  was  a  return  to  the  advantages  of  gono- 
chorism  through  the  development  of  almost  innumerable 
devices  tending  to  promote  frequent  crossing  between 
plants  of  the  same  and  nearly  related  species.  Some 
species  did  actually  return  to  true  structural  gonochorism, 
but  in  most  cases  other  means  of  obtaining  cross-fertiliza- 
tion were  developed.  There  was  no  advantage,  consider- 
ing their  sessile  mode  of  life,  in  relinquishing  the 
possibility  of  self-fertilization. 

Some  of  the  various  cross-fertilization  mechanisms 
utilized  are  very  reminiscent  of  those  of  animals.  Monce- 
cism,  the  production  of  male  and  of  female  flowers  on  the 
same  plant,  and  dichogamy,  the  maturation  of  the  male 
and  female  organs  at  different  times,  have  their  counter- 
parts in  the  other  kingdom.  So  also  the  physiological  phe- 
nomenon self-sterility  of  which  only  one  instance  is  known 
among  animals  is  very  common  among  plants.  Some 
hundred  or  so  species  distributed  throughout  thirty-five 
or  more  families  have  been  shown  to  be  self-sterile,  al- 
though the  true  number  is  probably  many  times  this 
figure.  Again  polygamy,  where,  in  addition  to  hermaph- 
roditic flowers,  either  male  or  female  flowers  are  devel- 

3 


34 


INBEEEDINQ  AND  OUTBREEDING 


oped,  has  its  analogue  in  the  complemental  males  charac- 
teristic of  the  Cirripedes. 

But  by  far  the  most  numerous  and  most  iateresting 
adaptations  for  cross-pollination  are  characteristic  of 
plants  alone.  These  are  the  thousands  of  structural  modi- 
fications which  utilize  external  agencies.  Wind  and  water 
have  not  been  despised,  but  the  real  servants — ^they  are 


FiQ.  8. — Adaptation  for  oroBa-pollination,  transference  of  pollen  by  insects.   (After  Kerner.) 

not  slaves  for  they  are  paid  for  their  services — are  the 
lower  animals  and  in  particular  the  insects. 

The  ideas  of  Darwin  resulting  in  the  tremendous  labors 
of  Miiller,Delpino,  Kerner,  Knuth  and  others  have  made  it 
no  longer  necessary  to  describe  the  facts  concerning  the 
dispersal  of  pollen  by  animals.^^**  ^^'^  The  subject  has 
been  so  fascinating  that  it  is  common  knowledge  how  the 
insects  are  attracted  to  flowers  by  odor  and  by  color ;  how 
they  are  rewarded  for  visits  by  nectar  and  by  pollen ;  how 


ANIMAL  AND  PLANT  REPRODUCTION   35 

provisions  are  made  to  use  them  as  pollen  carriers 
through  numberless  modifications  of  calyx,  corolla,  sta- 
mens and  pistil;  how  the  animals  themselves  have  devel- 
oped organs  for  extraction  of  food  or  for  attachment  to 
the  blossoms  (Fig.  8).  Perhaps  some  of  these  mutual 
adaptation  mechanisms  are  a  little  fanciful,  but  the  fact 
remains  that  actually  an  occasional  or  a  frequent  cross- 
poUination  is  secured  by  a  majority  of  our  100,000  or  more 
species  of  flowering  plants  by  means  of  insects,  and  the 
hundreds  of  mechanisms  by  which  it  is  obtained  are  wit- 
ness of  its  paramount  importance. 

The  thesis  of  this  chapter,  then,  is  simple.  The  whole 
trend  of  evolution  in  both  animals  and  plants  as  regards 
all  the  mechanisms  in  any  way  connected  with  reproduc- 
tion, has  been  such  as  to  provide  effectively  for  continuous 
descent.  In  the  midst  of  strenuous  competition  for  place, 
those  organisms  which  were  able  to  cross  with  others,  at 
least  occasionally,  held  such  an  advantage  over  those 
which  were  compelled  to  continue  through  one  single  line 
of  descent,  that  their  descendants  have  persisted  in  greater 
numbers.  They  have  dominated  the  organic  world.  Any 
satisfactory  interpretation  of  the  effects  of  inbreeding 
and  outbreeding  must  permit  a  reasonable  explanation  of 
this  situation. 


CHAPTER  III 
THE  MECHANISM  OF  EEPEODUCTION 

Theee  is  a  division  of  labor  in  all  the  higher  plants  and 
animals,  the  result  of  setting  apart  definite  tissues  for 
producing  germ  cells.  In  addition,  another  important 
matter  is  accomplished.  The  germ  cells  are  insulated 
from  ordinary  environmental  changes,  and  are  enabled  to 
go  through  a  very  exact  routine  of  processes  in  prepara- 
tion for  the  formation  of  the  new  organism — the  zygote. 

In  general  the  animal  body  or  the  sporophyte  of  the 
higher  plants  can  be  considered  as  a  double  organization. 
Various  parts  make  up  each  of  the  cell  units ;  but  of  them 
all  the  nucleus,  and  within  the  nucleus  the  chromosomes, 
seem  to  be  the  most  important.  Each  species  has  a  char- 
acteristic and  constant  number  of  these  bodies,  and  it  is 
their  distribution  which  parallels — and  probably  regu- 
lates— ^the  distribution  of  the  hereditary  differences 
within  a  species.  The  double  organization  of  the  bodies 
of  the  higher  organisms  is  dependent  upon  the  receipt  of 
one  set  of  these  chromosomes  from  each  parent.  And  it 
is  the  peculiar  method  by  which  these  chromosomes  are 
apportioned  to  the  gametes,  together  with  experiments  on 
the  actual  distribution  of  characters  in  the  generations 
succeeding  a  cross,  which  have  given  us  a  fairly  clear 
idea  of  heredity  as  a  mechanical  process. 

In  ordinary  cell  division  during  growth  each  chromo- 
some divides  longitudinally  so  that  both  daughter  cells 
apparently  receive  an  exact  half  of  the  chromatin,  al- 
though possibly  some  sort  of  a  special  apportionment  is 

36 


THE  MECHANISM  OF  REPRODUCTION      37 

made  in  the  segregation  of  particular  tissues.  But  when 
the  germ  cells  are  formed,  at  spermatogenesis  and  oogen- 
esis, the  chromosomes  unite  in  pairs,  a  process  technically 
known  as  synapsis,  and  at  division  one  member  of  each 
pair  passes  entire  to  one  of  the  two  new  daughter  cells, 
thus  reducing  the  number  of  the  chromosomes  in  the 
gamete  to  one-half  of  those  possessed  by  the  body  cells. 
Subsequently  there  is  an  equating  or  halving  division 
similar  in  appearance  to  the  cell  divisions  in  ordinary 
growth.  Four  gametes  are  thus  formed.  Leaving  out  of 
account  the  behavior  of  certain  chromosomes  believed  to 
control  the  distribution  of  sex,  there  is  good  evidence  that 
this  union  of  chromosome  pairs  at  synapsis  always  takes 
place  between  two  chromosomes,  one  of  which  had  been 
received  from  the  father  and  one  from  the  mother.  In 
other  words,  it  seems  clear  that  each  gamete  obtains  one 
of  each  hind  of  chromosome,  although  it  is  a  matter  of 
chance  whether  the  cell  receives  the  maternal  or  paternal 
representative  of  any  type.  Thus,  if  the  chromosomes 
of  the  body  cells  of  a  particular  species  are  six  in  number, 
and  we  represent  them  as  ABC  ahc,  regarding  A,  B,  and 
C  as  of  maternal  and  a,  b,  and  c  as  of  paternal  origin,  at 
synapsis  A  only  pairs  with  a,  B  with  b  and  C  with  c. 
This  procedure,  however,  will  yield  eight  types  of  gam- 
etes, ABC,  ABc,  AbC,  aBC,  Abe,  aBc,  abC,  and  abc,  siuce  it 
is  a  mere  matter  of  chance  which  daughter  cell  receives 
either  member  of  any  pair. 

In  spermatogenesis  four  sperms  are  formed  from  each 
immature  germ  cell,  but  in  oogenesis — the  maturation  of 
the  egg — only  one  functional  gamete  is  produced,  the 
other  three  being  aborted.  Nevertheless,  the  two  processes 


38 


INBREEDINa  A^D  OUTBREEDING 


are  similar  in  all  essential  features,  as  may  be  seen  in  Fig. 
9,  the  elimination  of  three  out  of  four  of  the  oocytes  taking 


SPSRMATOOKNBSIS 

I 


$pj5.jto. 


Spermatocyte 


Secondary 
— rwato- 
cytes 


Spertnato 


imr 


Spera- 
uosoa 


Multiplication 
p*riod 


056EN£SIS 

i 


.^ 


(^  h. 


Odgonia 


•■      Growth  period     -* 

Pairing  of  Chrorsosomes 
Reducing  division       J 


\ 


/    \ 


Primary  oocyte 


Secondary  oocyte 

(cvoft)  and  f iret 

polar  body) 


Mature  ovup  ^, 
and   poiar  bodies 


Mature  ovin 


WUn*  0  f)  1^^ 


.  mmber  cS 


Fio.  9. — Diagram  of  gametogenesis  showing  the  parallel  between  maturation  of  the  aperm 
cell  and  maturation  of  the  ovum.     (After  Guyer.) 

place  in  order  that  their  store  of  nutritive  materials  may 
go  to  make  one  large  egg. 

Fertilization  consists  in  the  fusion  of  one  egg  with  one 
sperm,  thus  bringing  back  the  double  number  of  chromo- 


THE  MECHANISM  OP  REPKODUCTION      39 


somes  characteristic  of  the  body  cells  (Fig  10).  and  since 
it  is  a  matter  of  chance  what  gametes  unite,  such  gametic 
differences  as  we  have  illustrated  would  give  a  possibility 
of  obtaining  8  x  8  or  64  types  of  zygotes. 


FiQ.  10. — Diagram  to  illustrate  fertilization;  & ,  male  pronucleus;  9  ,  female  pro- 
nucleus; observe  that  the  chromosomes  of  maternal  and  paternal  origin,  reepectively,  do 
not  fuse.    (After  Guyer.) 

The  mechanism  of  the  process  of  gametogenesis  and 
fertilization  in  animals  need  not  concern  us  further  here. 
We  must  speak  of  the  process  in  the  seed  plants,  however, 
for  a  rather  odd  phenomenon  occurs  there  to  which  there 
will  be  occasion  to  refer  later. 


40 


INBEEEDING  AND  OUTBREEDING 


Reduction  of  the  chromosomes  takes  place  in  plants 
just  as  it  does  in  animals,  but  the  introduction  of  a  gamete 
generation,  the  gametophyte,  complicates  matters.  In 
the  seed  plants,  pollen  mother  cells  are  produced  in  the 
anthers  of  the  flower  which  go  through  precisely  the 
same  divisions  as  in  animal  spermatogenesis  (Fig.  11). 
But  each  of  the  four  nuclei  thus  produced  divides  during 


Fig.  11. — Formation  of  pollen  grains  in  the  lily.  B,  stages  in  the  formation  of  pollen 
grains  in  a  group  of  four  (tetrad)  within  the  pollen  mother  cell;  C,  mature  pollen  grain 
with  early  stages  in  the  development  of  the  male  gametophyte;  t,  tube  nucleus;  g,  generative 
nucleus.     (After  Bergen  and  Davis.) 

the  formation  of  the  pollen  grain,  forming  a  generative 
and  a  tube  nucleus.  The  tube  nucleus  it  is  that  germinates 
and  passes  down  the  style  when  the  pollen  grain  falls  on 
a  ripe  stigma.  During  this  period  of  pollen  tube  growth 
the  generative  nucleus  passes  down  through  it  toward  the 
ovule,  and  while  so  doing  divides  again,  leaving  two  nuclei 
each  wdth  a  function  to  perform.  One  fuses  with  the  egg 
and  the  other  with  the  so-called  endosperm  nucleus,  com- 


THE  MECHANISM  OF  REPRODUCTION      41 


pleting  in  this  manner  the  peculiar  double  fertilization 
characteristic  of  the  angiosperms. 

In  the  meantime,  the  egg  and  the  endosperm  yiucleus 
have  been  prepared  by  the 
accessary  cell  divisions  of  the 
female  gametophyte.  The 
reduction  division  occurs  in 
the  usual  manner,  and  as  in 
animals  three  of  the  cells  are 
absorbed,  leaving  a  single 
one  to  provide  for  the 
hereditary  succession.  Its 
container  enlarges  and  be- 
comes the  embryo  sac,  while 
the  cell  itself  typically  goes 
through  three  cell  divisions 
resulting  in  the  formation  of 
eight  nuclei.  Any  of  these 
nuclei  may  become  the  egg, 
but  generally  the  egg  can  be 
recognized  by  its  position 
(Fig.  12).  Two  others  from 
among  these  nuclei  fuse  to- 
gether and  become  the  endo- 
sperm nucleus,  which  in  turn 
fuses  with  the  second  male 
nucleus  and  by  succeeding 
cell  divisions  forms  the  en- 
dosperm of  the  seeds,  the  function  of  which  is  to  furnish 
food  for  the  young  plant,  the  embryo.  Thus,  if  we 
represent  the  chromosome  complex  of  the  gametes  by  x, 
the  embryo  is  2x,  and  the  endosperm  3a;. 


Fertilization  in  the  embryo  sac 
e,  egg;  fs,  first  ppcrm;    pv, 
fused    polar    nuclei  ;    ss,     second    sperm. 
(After  Bergen  and  Davis.) 


Fia.  12 
of   the   lily 


42 


INBEEEDING  AND  OUTBREEDING 


It  is  clear  from  this  short  description  of  gametogenesis 
and  fertilization  that  the  processes  in  plants  and  in  ani- 
mals are  identical  in  what  we  deem  to  be  the  essential 
features,  the  behavior  of  the  chromosomes.  If  one  visual- 
izes the  behavior  of  hereditary  characters  in  crosses  in 
which  the  parents  differ  as  the  result  of  the  operation  of 
potential  factors  carried  by  these  bodies,  he  can  correlate 


i 


J 


•V 


'■iW  T 


Fig.  13. — Entrance  of  the  spermatozoon  through  the  membrane  of  the  egg  of  a  etar- 
fish  giving  an  idea  of  the  difference  in  size  between  the  sperm  and  the  egg.  (Wilson's  "The 
Cell."    Courtesy  Macmillan  Co.) 

every  fact  thus  far  discovered,  with  the  exception  of  a 
few  isolated  cases  found  in  plants  where  particular  char- 
acteristics appear  to  be  distributed  by  the  cytoplasm  lying 
outside  of  the  nucleus.  Not  only  can  the  distribution  of 
ordinary  characters  be  interpreted  as  functions  of  the 
chromosomes,  but  the  distribution  of  the  sexes  as  well. 
There  is  reason  to  think  the  behavior  of  the  sex-control- 
ling chromosomes  may  perhaps  occasionally  be  influenced 
by  external  conditions,  but  sex  itself  is  determined  by  the 


THE  MECHANISM  OF  REPEODUCTION 


43 


behavior  of  particular  chromosomes  of  which  we  have  not 
hitherto  spoken  (Fig.  14). 

The  evidence  in  favor  of  this  view  of  the  determination 
of  sex  at  the  time  of  fertilization  through  the  chromosome 
complex  is  from  several  very  different  sources. 

First,  there  is  the  phenomenon  of  multiple  births 

Protevior     cf 


••• 


B 


•••••• 

Fig.  14. — Diagram  showing  the  distribution  of  the  sex  chromosome  in  Protenor.     (After 

Morgan.) 

among  mammals.  In  general,  animals  in  which  this  is 
the  rule,  bear  both  males  and  females,  through  all  of  the 
individuals  must  have  been  under  the  same  environmental 
conditions.  There  are  multiple  births,  however,  in  which 
the  young  are  invariably  of  the  same  sex.  Such  is  the 
case  with  those  remarkably  similar  human  twins  kno^^^l  as 
identical  twins.  Such  is  the  case  with  the  four  young  in 
each  litter  of  the  nine-banded  armadillo  (Fig.  15).    Now 


44 


INBREEDINa  AND  OUTBREEDING 


it  can  be  shown  that  in  these  two  and  other  similar  in- 
stances, the  several  young  are  the  product  of  a  single 
fertilized  egg  which  so  develops  as  to  form  two  or  four 
complete  individuals.  If  sex  was  determined  after  fer- 
tilization, one  might  expect  a  random  sample  of  the  two 
sexes  here,  but  this  is  not  the  case. 


I 


FiQ.  15. — Identical   quadruplets  in   the    nine-banded  armadillo. 

Doncaster.) 


(After   Newman  from 


The  chief  support  of  this  idea  of  sex  determination, 
however,  comes  from  the  microscope  and  the  breeding 
pen.  In  an  ever  increasing  number  of  species,  possibly 
including  man  himself,  it  has  been  found  that  besides  the 
regular  paired  chromosomes,  the  autosomes,  there  is  a 
single  chromosome  or  possibly  a  chromosome  group  com- 
monly known  as  the  x-chromosome,  whose  behavior  in  cell 
division  is  somewhat  different  from  the  others,  and  whose 


THE  MECHANISM  OF  REPRODUCTION      45 

distribution  absolutely  parallels  the  distribution  of  sex. 
There  are  two  types.  In  the  males  of  animals  of  Type  A, 
which  includes  numerous  flies,  beetles,  grasshoppers  and 
bugs  from  among  the  insects,  as  well  as  representatives 
from  several  orders  of  mammals,  a  single  x-chromosome 
is  present  in  addition  to  the  regular  chromosome  pairs, 
and  for  this  reason  two  kinds  of  spermatozoa  are  pro- 
duced at  spermatogenesis  in  equal  numbers,  those  pos- 
sessing the  extra  element  and  those  without  it.  In  the 
females,  on  the  other  hand,  two  of  these  elements  are 
present  and  the  eggs,  therefore,  always  possess  it.  Thus, 
on  fertilization,  half  of  the  resulting  young  have  two 
x-chromosomes  and  these  become  females,  while  half  ha^^e 
but  one  and  become  males. 
Diagrammatically  it  is  this : 

Ovum  with  x  fertilized  by  sperm  mth  x  =  female. 
Ovum  with  x  fertilized  by  sperm  without  x  -  male. 

In  some  other  cases  (Type  B),  the  eggs  are  di- 
morphic, while  the  sperm  are  all  alike,  but  the  result 
is  the  same ;  the  sex  distribution  follows  the  chromosome 
differentiation. 

In  dioecious  plants  there  is  some  evidence  of  a  similar 
condition.  Strasburger  ^^^  found  in  one  of  the  liverworts, 
Sphaerocarpus,  where  the  four  spores  produced  by  a  single 
spore  mother  cell  hang  together  and  each  such  tetrad 
could  be  planted  separately,  that  invariably  two  males  and 
two  females  were  produced.  More  recently  Allen  ^  has 
presented  evidence  of  an  x-chromosome  in  this  genus.  His 
discovery  was  made  with  material  of  the  species  Spliwro- 
carpus  Donnellii,  but  it  has  been  corroborated  by  one  of 
his  students  working  with  Sphccrocarpus  texanus. 


f^ 


46  INBREEDING  AND  OUTBREEDING 

Again,  in  the  dioecious  moss,  Funaria,  the  Marchals;^^^ 
by  a  remarkable  series  of  regeneration  experiments,  have 
proven  the  determination  of  sex  at  the  reduction  division. 
Each  spore  was  found  to  contain  the  potentialities  of  but 
one  sex,  but  in  the  sporophyte  they  demonstrated  the  po- 
tentialities of  both  sexes  by  inducing  direct  aposporous 
development  of  gametophytes,  which  proved  to  have  both 
antheridia  and  archegonia,  the  organs  of  both  sexes. 

The  situation  in  hermaphroditic  plants  and  animals 
is  not  so  clear.  Particularly  in  plants  the  peculiar  life 
history  with  the  introduction  of  alternation  of  generations, 
makes  experimental  work  exceedingly  difficult.  Further- 
more, there  are  many  species  of  animals  where  the  sex 
ratio  is  nowhere  near  equality  and  where  both  external 
and  internal  conditions  undoubtedly  do  have  marked  in- 
jfluence,  but  in  such  a  fundamental  phenomenon  we  can 
hardly  believe  these  difficulties  are  insurmountable  or 
will  lead  to  any  radically  different  interpretation  of  the 
problem.  "Where  there  is  such  clear  evidence  fromvery  dif- 
ferent modes  of  attack  and  upon  species  so  unrelated  one 
is  constrained  to  believe  the  obstacles  to  a  unified  theory 
are  only  superficial.  This  is  particularly  true  since  there 
is  another  line  of  experimental  evidence  in  favor  of  the 
determination  of  sex  by  the  chromosomes.  Our  whole 
evidence  on  inheritance,  in  f  a-ct,  is  linked  up  with  chromo- 
some distribution,  so  that  the  easiest  way  to  visualize  the 
process  is  by  supposing  that  the  individual  potentialities, 
the  factors,  which  cooperate  in  the  development  of  plant 
and  animal  characters,  are  disposed  in  a  definite  manner 
in  the  chromosomes,  as  we  shall  see  in  the  next  chapter. 
The  particular  discoveries  which  demand  our  attention  in 
connection  with  the  phenomenon  of  sex,  however,  are 


THE  MECHANISM  OF  REPKODUCTION      47 

those  regarding  characters  commonly  known  as  sex- 
linked,  whose  distribution  can  be  accurately  predicted 
if  we  assume  they  are  definitely  coupled  with  the 
sex  determiner. 

Such  a  character  is  hereditary  color-blindness  in  maji, 
a  condition  in  which  the  affected  individual  cannot  dis- 
tinguish between  red  and  green.  It  is  far  commoner  in 
man  than  in  women,  and  its  inheritance  is  so  peculiar  that 
it  often  seems  to  skip  a  generation. 

A  color-blind  man  married  to  a  normal  woman  will 
have  only  normal  children  of  either  sex.  The  sons  will 
never  have  color-blind  progeny  by  women  with  nonnal 
vision,  but  the  daughters,  though  married  to  normal  men, 
will  transmit  color-blindness  to  one-half  of  their  sons. 
If,  moreover,  a  daughter  mates  with  a  color-blind  man, 
as  might  frequently  happen  in  marriage  between  cousins, 
on  the  average  one-half  of  her  daughters  as  well  as  one- 
half  of  her  sons  will  be  abnormal. 

This  interesting  and  apparently  complicated  inheri- 
tance is  really  very  simple  if  we  merely  assume  that  the 
sex  chromosomes  of  the  color-blind  individuals  also  carry 
the  determiner  for  color-blindness.  Fig.  16  shows  what 
would  be  expected.  Eepresenting  the  normal  vision  by 
boldface  type  and  color-blindness  by  outline  we  see  first 
the  result  of  mating  a  normal  woman  with  a  color-blind 
man.  Since  all  of  her  sex-cells,  when  matured,  contain 
one  normal  x-element,  and  since  the  sex-cells  of  the 
male  are  of  two  kinds,  half  containing  an  abnormal  or 
color-blind  determining  x-element  and  half  containing  no 
x-element  whatever,  it  is  obvious  that  the  sons  must  re- 
ceive their  x-element  only  from  their  mother  and  the 
daughters  must  receive  one  of  their  x-elements  from  their 


48 


INBEEEDING  AND  OUTBREEDING 


father.     The  sons,  therefore,  cannot  be  color-blind  and 
cannot  transmit  color-blindness,  but  the  daughters,  though 


(Normal) 


VaU  tin* 
(Color  bUDd)| 


of  parenVs 


o(  paresU 


fiodjr-cellt 
ot  chlldreo 


Sex-cellt 
ot  cbildroa. 


Body-cells 
of  ^rand- 
«bildrea 


FiQ.  16. — Diagram  illustrating  the  inheritance  of  a  sex-linked  character  such  as 
color-blindness  in  man  on  the  assumption  that  the  factor  in  question  is  located  in  the  sex 
chromosome.  The  normal  sex  chromosome  is  indicated  by  a  black  X,  the  one  lacking 
the  factor  for  color  perception,  by  a  light  X.  It  is  assumed  that  a  normal  female  is  mated 
with  a  color-blind  male.     (After  Guyer.    Courtesy  Bobbs  Merrill  Co.) 

they  will  not  be  color-blind  themselves  because  one  normal 
x-element  is  sufficient  to  determine  normal  vision,  will 
produce  defective  x-elements  in  one-half  of  their  ova, 


THE  MECHANISM  OF  REPRODUCTION      49 

and  for  this  reason  will  transmit  color-blindness  to  one- 
half  of  their  sons  by  a  normal  man,  as  will  be  seen  by 
following  out  the  fourth  and  fifth  columns  in  the  diagram. 
An  egg  containing  the  normal  x-element  can  meet  a  sper- 
matozoon carrying  an  x-element  and  thus  produce  a 
daughter,  or  it  may  meet  a  spermatozoon  with  no  x-ele- 
ment and  thus  produce  a  son;  but  in  either  case  the  chil- 
dren will  have  normal  vision.  On  the  other  hand,  an  egg 
containing  a  defective  x-element  will  by  similar  fertiliza- 
tions result  either  in  a  normal-visioned  daughter,  who  will 
carry  color-blindness  in  half  of  her  ova,  or  in  a  son  who 
will  be  color-hlind. 

Such  a  scheme  of  interpretation  might  seem  quite 
visionary  were  it  not  for  the  fact  that  similar  types  of 
inheritance  occur  in  many  of  the  lower  animals.  By  care- 
fully controlled  experiments  with  them  it  has  been  proven 
beyond  a  doubt. 


CHAPTER  IV 
THE  MECHANISM  OF  HEEEDITY 

The  scientific  era  in  the  investigation  of  heredity  be- 
gan in  the  latter  half  of  the  nineteenth  century  with  the 
57ork  of  Galton  and  of  Mendel.  Both  enthusiastic  and 
competent  investigators,  their  efforts  made  with  differ- 
ent material  and  from  diverse  points  of  view,  did  not 
fare  the  same.  Galton  measured  the  inheritance  of  groups 
of  individuals  by  their  resemblance  to  their  progenitors 
and  failed  because  his  method  could  not  take  into  account 
the  true  relationship  between  the  germinal  constitution 
and  the  body  characters  of  an  individual ;  Mendel  deter- 
mined the  inheritance  of  a  single  organism  by  making  the 
characters  of  its  progeny  the  criterion  and  succeeded. 
Without  knowledge  of  the  cell  mechanism  of  gameto- 
genesis  and  fertilization,  Mendel  described  the  results  of 
his  hybridization  experiments  in  terms  which  agreed  pre- 
cisely with  these  later  discoveries  in  the  field  of  cytology. 
Mendelian  heredity  has  proved  to  be  the  heredity  of  sex- 
ual reproduction:  the  heredity  of  sexual  reproduction 
is  Mendelian. 

Progress  in  the  study  of  heredity  through  investiga- 
tions patterned  after  Mendel's  model  has  been  so  great 
that  the  subject  now  forms  an  important  sub-division  of 
general  biology — Genetics.  The  details  of  the  subject 
have  outgrown  the  limits  of  a  single  volume,  and  a  knowl- 
edge of  the  generalities  is  no  longer  confined  to  the  pro- 
fessional biologist.     For  such  reasons  we  propose  to 

60 


THE  MECHANISM  OF  HEREDITY  51 

discuss  here  only  the  broader  relationships  of  Mendelian 
heredity  to  the  behavior  of  the  chromosomes,  since  this 
phase  must  be  emphasized  as  a  basis  for  correlating  the 
facts  from  Nature's  experiments  on  inbreeding  and  out- 
breeding with  the  results  from  the  experiments  made 
by  man. 

The  Mendelian  method  of  studying  heredity  consists 
essentially  in  crossing  forms  which  differ  by  well-defined 
characteristics  and  in  following  the  distribution  of  these 
characteristics  separately  and  quantitatively  in  the  suc- 
ceeding generations.  If  a  wheat  with  a  long  lax  head  or 
spike  is  crossed  with  one  having  a  short  dense  spike  the  F, 
(first  filial)  generation  bears  intermediate  spikes.  The 
Fi  generation,  self-fertilized,  however,  yields  all  three 
types — long,  intermediate  and  short  spikes — in  the  F2 
generation ;  and  in  large  numbers  these  types  bear  a  con- 
stant ratio  to  each  other  in  the  proportion  1  long  spike : 
2  intermediate  spikes :  1  short  spike.  Nor  is  this  all.  The 
long-spiked  plants  all  breed  true  to  long  spikes,  the  short- 
spiked  plants  all  breed  true  to  short  spikes,  while  tlv 
plants  bearing  intermediate  spikes  again  produce  the 
ratio  exhibited  by  the  F^  generation.  Diagrammatically 
the  result  of  the  cross  is  as  follows : 

Pj        Long  spikes  x  Short  spikes 

I 
Fj  Intermediate  spikes 

^-^         II 

F„     Long  spikes      Intermediate  spikes       Short  spikes 

I  ^     W    ^^  I 

Fg      Long  spikes  Long  spikes    Inter-    Short  spikes    Short  spikes 

mediate 
spikes 


52 


INBREEDING  AND  OUTBREEDING 


If  the  description  of  the  dual  nature  of  the  cells  of 
plants  and  animals  and  the  result  of  gametogenesis  is 
recalled,  the  reason  for  the  production  of  the  ratio  of  1 
long  spike :  2  intermediate  spikes :  1  short  spike  in  the  F2 
generation  is  plain.  Furthermore,  it  is  clear  why  the 
types  like  the  grandparents  breed  true  and  the  type  like 
the  hybrid  F^  generation  does  not  breed  true. 

The  long-spiked  wheat  has  received  the  factor  for  long 
spikes,  the  something  in  the  germ  cell  that  stands  for  the 
production  of  long  spikes,  from  hoth  of  its  parents ;  there- 


Feviale  gantete 


\likU  gamete 


Fia.   17. — Diagram  showing  the  union  of  like  gametes. 

fore  it  breeds  true  to  long  spikes.    The  gametes  which  it 
produces  all  bear  the  factor  for  long  ears. 

The  diagram  illustrating  the  fusion  of  the  parental 
gametes,  shows  why  the  long-spiked  wheat  produces 
gametes,  each  of  which  bears  the  factor  for  long  spikes. 
If  the  letter  8  is  substituted  for  the  letter  L  in  the  dia- 
gram, the  same  illustration  holds  for  the  short-spiked 
wheat.  But  what  happens  when  the  long-spiked  variety 
is  crossed  with  the  short-spiked  variety?  A  gamete  bear- 
ing L  fuses  with  a  gamete  bearing  ^S'  and  a  zygote  LS  is 
formed.  The  interaction  of  the  factors  L  and  S  produces 
an  F^  plant  bearing  intermediate  ears.    "When  this  hybrid 


THE  MECHANISM  OF  HEREDITY 


53 


comes  to  produce  gametes  they  bear  either  the  one  or  the 
other — and  never  both — of  these  factors.  In  other 
words,  the  germ  cells  (both  male  and  female)  of  the  hy- 
brid are  half  of  them  L  and  half  of  them  S.  When  the 
F^  generation  is  selfed,  therefore,  it  is  a  matter  of 
chance  which  of  these  geiTa  cells  meet  to  form  zygotes. 
If  a  large  progeny  is  produced,  there  will  be  a  ratio  of 

1|L|L|  :  2|L|/S'| :  IIaS'IiS'I,  and  since  the  formulas    L\L\  and 


S\S\  are  like  the  zygotic  formulae  of  the  long-spiked  and 
short-spiked  parents,  respectively,  the  plants  that  they 
produce  will  be  long-spiked  and  short-spiked,  as  the  case 
may  be,  and  will  breed  true  to  that  character.  The  inter- 
mediates, however,  having  been  produced  by  zygotes 


L  S  lilie  the  F^  generation,  will  behave  in  the  same  man- 


ner when  selfed. 


That  the  ratio  will  be  approximately  1  L\L    :  2  L  S\ 


1  8  S  is  plain  if  one  thinks  for  a  moment  what  the  result 


would  be  if  a  thousand  tickets  bearing  the  letter  L  and  a 
thousand  tickets  bearing  the  letter  S  were  shuffled  up  in  a 
hat  and  drawn  out  in  pairs,  replacing  the  pair  each  time 
after  drawing  and  recording.  Suppose  the  first  member 
of  the  pair  represents  the  egg  cell;  the  chances  are  % 
that  it  will  be  L  or  S.  The  second  member  of  the  pair 
represents  the  male  cell  and  the  chances  are  likewise  % 
that  it  will  be  L  or  ^S'.  Therefore,  when  L  is  the  first  mem- 
ber of  the  pair,  half  of  the  time  the  zygote  formed  will  be 


L  L    and  half  of  the  time  it  will  be    L  S  ,     Likewise, 


when  8  is  the  first  member  of  the  pair,  zygotes  \S\L\ 


and   S  S|  will  be  formed  in  equal  quantities.    Combining 


these  possibilities,    the  ratio  1|-^|L|  :  2|L|5'|  :  1|<S'|<S'|  is 


54 


INBREEDING  AND  OUTBREEDING 


obtained.    Diagrammatically  the  expected  production  of 
gametes  and  zygotes  is  as  follows : 


^  gametes 


Jj  zygote 


Eggs 


R  gametes 


L       S 

s  ' 

S  S  L  S 


L  S 


Pa  2ygote 


LL  LS  SL  SS 

I I         I 1 1  I 1 — 


Fia.  18. — Diagram  to  illuatrate  Menddiam  in  a  crosa  between  long-spiked  and  short-spiked 

wheat. 

Let  us  express  the  whole  matter  mathematically. 
The  chance  of  an  event  happening  in  an  infinite  num- 
ber of  trials  is  expressed  by  a  fraction  of  which  the 
numerator  is  the  number  of  favorable  ways  and  the  de- 
nominator the  whole  range  of  possibilities,  both  favor- 
able and  unfavorable,  if  each  is  equally  likely  to  occur. 
Hence,  certainty  is  expressed  in  the  figure  1.  Further- 
more, the  chance  that  two  or  more  independent  events 
will  happen  together  is  the  product  of  their  respective 
chances  of  happening.    The  chance  of  an  L  egg  meeting 


THE  MECHANISM  OF  HEREDITY  55 

an  L  sperm  is  %  x  %  =  14,  and  the  chance  of  an  L  egg 

meeting  an  8  sperm  is  %  x  1/2  =  %•  Similarly,  the  chance 

of  an  S  egg  meeting  an  L  sperm  is  %  ^^^  of  an  L  egg 

meeting  an  S  sperm  %.     The  sum  of  the  possibilities 

is  then 

L  +L  =  1/4 
6'4-L  =  l/2 
S-{-S  =1/4 

One  important  thing  to  be  remembered,  however,  is 
that  the  law  of  chance  expresses  the  probability  when  in- 
finitely large  numbers  are  concerned.  Wlien  small  num- 
bers are  dealt  with,  departures  from  the  expected  ratios 
are  obtained.  It  is  a  matter  of  common  sense  that  al- 
though the  chances  of  throwing  heads  with  a  penny  are  1/2, 
yet  in  a  small  mmiber  of  throws  exactly  %  of  them  may 
not  be  heads.  A  regular  ratio  of  these  departures  is  to  be 
expected  which  accords  with  the  law  of  error. 

Typically,  experiments  such  as  this  are  the  basis  for 
MendePs  Laws  of  Inheritance,  the  first  of  which  may  be 
stated  as  follows :  U-nit  factors  contributed  by  tivo  parents 
hailing  definite  roles  in  the  development  of  characters, 
separate  in  the  germ  cells  of  the  offspring  without  having 
influenced  each  other.  Although  Mendel  himself  knew  of 
no  mechanism  by  which  such  a  process  could  take  place, 
although  his  theory  was  evolved  wholly  as  a  fitting  inter- 
pretation of  the  facts  obtained  by  breeding,  what  can  be 
more  reasonable  than  to  suppose  that  the  germ  cell  factors 
reside  in  the  chromosomes  and  that  the  separation  of  the 
chromosomes  at  the  reduction  division  in  gametogenesis 
furnishes  the  segregation  of  factors  required? 

The  interaction  of  a  pair  of  homologous  factors  in  a 
hybrid — allelomorphs  they  are  called — does  not  always 


56  INBEEEDING  AND  OUTBREEDING 

result  in  the  production  of  an  intermediate.  Often  the 
action  of  one  factor  dominates  the  action  of  the  other, 
either  by  masking  it  or  by  inhibiting  its  operation.  When 
this  occurs  the  dominated  character  recedes  from  sight  in 
the  Fi  generation  and  the  ratio  in  the  F2  generation  is 
3  dominant :  1  recessive.  But  since  only  one-third  of  these 
dominants  breed  true  *  and  two-thirds  behave  as  did  the 
Fi  generation,  the  results  are,  therefore,  comparable  with 
those  illustrated  by  the  wheat,  and  the  phenomenon  of 
dominance  is  a  mere  detail. 

Mendel  was  not  content  with  experiments  in  which 
only  one  pair  of  differentiating  characters  was  concerned. 
He  made  crosses  between  varieties  of  the  garden  pea 
which  differed  by  two  and  by  three  allelomorphic  pairs  of 
characters,  and  was  rewarded  for  his  perseverance  by 
discovering  a  second  law,  usually  known  as  the  Law  of 
Recombination.  This  laiv  states  that  two  or  more  allelo- 
morphic pairs  of  factors  may  segregate  independently 
and  may  recomhi<ne  in  all  the  combinations  possible  gov- 
erned by  chance  only. 

Though  thousands  of  characters  have  been  investi- 
gated since  MendePs  time  one  cannot  improve  on  the 
original  classic  as  illustrative  material  for  explanation 
of  dihybrid  heredity. 

With  two  pairs  of  characters  he  designated  the  factors 
representing  the  dominant  characters  as  A  and  B  and  the 
factors  representing  the  recessive  characters  as  a  and  b. 
The  characters  in  the  varieties  crossed  were  as  follows : 

Seed  parent  (  A  form  round  Pollen  Parent  J  a  form  wrinkled 

AB        }  B  cotyledon  yellow  ab  }  b  cotyledon  green 

a  When  the  factor  for  a  character  has  been  received  from  both  parents 
the  organism  is  said  to  be  homozygous  for  it;  if  it  has  been  received  from 
only  one  parent  the  individual  is  heterozygous  or  hybrid  for  it. 


THE  MECHANISM  OF  HEREDITY  57 

When  these  two  forms  were  crossed  all  of  the  hybrid's 
seeds  appeared  round  and  yellow  (AB)  like  those  of  the 
seed  parent;  that  is,  the  round  character  and  the  yellow 
character  were  each  dominant. 

When  these  F\  seeds  were  sown  and  the  plants  self- 
fertilized,  four  kinds  of  seeds  appeared  in  the  progeny 
in  the  four  combinations  that  were  possible,  with  the 
following  numbers  of  each : 

AB  round  and  yellow  315 

aB   wrinkled   and   yellow  101 

Ab  round  and  green  108 

ab  wrinkled  and  green  32 

These  figures  stand  approximately  in  the  relation  of 
9AB  to  SaB  to  SAb  to  lah.  The  forms  appeared  to 
belong  to  but  four  homogeneous  classes  (phenotypes). 
This  was  due  to  the  phenomenon  of  dominance  masking 
the  difference  between  homozygotes  and  heterozygotes. 
By  their  behavior  in  the  next  generation  they  were  found 
to  belong  to  nine  really  different  classes. 

From  the  round  yellow  seeds  (apparently  AB)  were  obtained  by 
self-fertilization : 

(1)  A  ABB    seeds  all  round  and  yellow  38 

(2)  AABb     seeds  all  round,  yellow  and  green  65 

(3)  AaBB    seeds  all  yellow,  round  and  wrinkled        60 

(4)  AaBb    seeds  round  and  wrinkled,  yellow 

and  green  138 

From  the  round  and  green  seeds  (apparently  Ab)  were  obtained: 

(5)  AAbb  seeds  all  round  and  green  35 

(6)  Adbb    seeds  all  green,  round  and  wrinkled        67 

From  the  Tvninkled  and  yellow  seeds  (apparently  aB)  were  obtained : 

(7)  aaBB    seeds  all  wrinkled  and  yellow  28 

(8)  aaBb    seeds  all  wrinkled,  yellow  and  green        67 

From  the  wrinkled  and  green  seeds  (apparently  ab)  were  obtained: 

(9)  aabb     seeds  all  wrinkled  and  green  30 


58  INBREEDING  AND  OUTBREEDING 

The  total  number  of  plants  in  tlie  F^  generation  is  not 
quite  the  same  as  the  F2  generation  due  to  seed  not 
germinating  or  plants  dying,  but  it  is  plain  that  the  ratio 
of  9AB  :  3aB  :  3Ab  :  lab  obtained  in  the  F^  generation 
is  made  up  of  the  following  actual  classes : 


9  apparently  AB  made  up  of 


3  apparently   Ab    made   up   of 


fl  AABB 
2  AaBB 
2  AABh 
4  AaBb 

1  AAhb 

2  Aabb 


a  auBB 
3  apparently   aB   made   up   of  jo  oaBb 

1  apparently   ah    made   up    of  1  aabb 

The  four  classes  AABB,  aaBB,  AAhh  and  aabh,  hav- 
ing each  factor  in  the  duplex  condition  bred  true;  the 
heterozygous  condition  of  one  or  both  allelomorphs 
in  the  remainder  was  shown  by  the  character  of  the 
F^  progeny. 

In  order  to  visualize  these  facts,  or  in  truth  the  facts 
from  any  number  of  pairs  of  allelomorphs  in  independent 
Mendelian  inheritance,  one  has  only  to  recall  that  pairs 
of  homologous  chromosomes — one  paternal  and  one  ma- 
ternal— meet  during  the  process  of  gametogenesis  and 
one  or  the  other  of  each  pair  passes  to  either  daughter 
cell.  The  factors  A  and  a  lie  in  corresponding  loci  in 
one  pair  of  chromosomes,  the  factors  B  and  b  lie  in  cor- 
responding loci  in  a  second  pair  from  among  the  seven 
paired  chromosomes  of  the  garden  pea.  Thus  gametes 
bearing  the  factors  AB,  Ab,  aB  and  ab  will  be  formed  in 


THE  MECHANISM  OF  HEREDITY 


59 


equal  quantities  in  both,  the  egg  cells  and  the  pollen  cells, 
as  is  shown  by  the  accompanying  diagram. 

I  H 


a 
B 

A 

b 

FiQ.  19. — Diagram  to  illuBtrate  gamete  formation  in  a  dibybrid  in  independent  inberitanoe. 

It  is  clear  that  if  this  process  occurs  in  both  male  and 
female  germ  cells,  and  these  germ  cells  unite  by  chance, 
the  result  is  that  obtained  in  the  breeding  experiments. 
The  posssible  matings  can  be  shown  graphically  as  follows : 


A    B 


A     b 


Eggs 


a     B 


a      b 


A    B 


Sperms 
A    b  a    B 


a    b 


A    B 

A    B 

A    B 

A    B 

A    B 

A     b 

a    B 

a    b 

A    b 

A     b 

A    b 

A    b 

A    B 

A     b 

a     B 

a     b 

a    B 

a     B 

a     B 

a    B 

A    B 

A    b 

a     B 

a    b 

a     b 

a     b 

a     b 

a    b 

A    B 

A    b 

a     B 

a     b 

60  INBREEDING  AND  OUTBEEEDING 

A  large  series  of  results  on  independent  Mendelian 
heredity  obtained  during  the  past  eighteen  years  by 
numerous  biologists  can  be  interpreted  in  the  same  man- 
ner. At  first  glance  many  of  their  results  appeared  to  be 
either  very  complex  or  very  irregular,  but  one  by  one 
they  were  shown  to  be  just  as  simple  as  the  cases  given. 
Considering  only  instances  which  may  be  interpreted  by 
two  factors,  for  example,  we  have  i^g  ratios  of  12:  3: 1, 
9:7,  9:3:4  and  13 :  3.  But  these  are  not  difficult  to  ana- 
lyze. They  are  simply  the  ordinary  9:3:3:1  ratios  in 
which  part  of  the  terms  are  combined  in  various  ways. 
For  example,  the  F2  ratio,  when  certain  black  beans  are 
crossed  with  certain  white  beans,  is  12  black  :  3  yellow  :  1 
white.  Clearly  this  is  because  black  +  yellow  {AB)  is  in 
appearance  not  different  from  black  {A)  alone.  A  purple 
sweet  pea  crossed  with  a  certain  white  variety  segregates 
in  a  ratio  of  9  purple :  7  white.  This  result  is  easily  ex- 
plained by  assuming  that  the  purple  color  only  appears 
when  a  color  base  factor,  A^  is  present  in  connection  with 
a  color-producing  factor,  B,  The  last  three  terms  of  the 
dihybrid  ratio,  '^AB  :  ?taB  :  lah,  therefore,  are  alike  in 
appearance.  This  assumption  has  been  proved  to  be  cor- 
rect in  other  ways.  Again  if  a  black  variety  of  rat,  mouse, 
guinea-pig  or  rabbit  be  crossed  with  a  white  variety  car- 
rying a  factor  for  ticking  the  hair  with  yellow,  known  as 
agouti,  the  segregating  ratio  is  9  agouti  :  3  black  :  4  white. 
The  reason  for  the  combination  of  classes  Ah  and  ah  is 
because  the  agouti  factor  does  not  show  except  in  the  pres- 
ence of  color  {B),  Finally,  a  factor  A  which  inhibits  the 
action  of  B  and  therefore  makes  AB  and  Ah  resemble  ah, 
gives  the  peculiar  ration  13 :  3.    Crossing  a  certain  race  of 


THE  MECHANISM  OF  HEREDITY  61 

white  fowls  with  colored  races,  for  instance,  gives  the 
ratio  13  white :  3  colored. 

It  is  evident,  if  one  runs  over  these  examples  and  ^vorks 
out  all  the  possibilities  involved,  he  will  find  that  two 
white  races  of  sweet  pea,  when  crossed,  will  give  purples 
in  the  F^  generation,  a  white  race  of  guinea-pigs,  crossed 
with  a  black  variety,  will  give  all  agouti,  etc.  Such  curi- 
ous results  are  actually  obtained.  They  are  quite  simple, 
and  their  whole  heredity  may  be  visualized  by  the  use  of 
the  same  chromosome  scheme  as  given  above.  Of  course, 
some  of  them  require  the  assumption  of  differences  in 
more  than  two  allelomorphic  factors,  but  this  can  be  done 
by  remembering  that  additional  factor  pairs  follow  the 
same  mathematical  scheme  as  do  one  or  two  pairs. 

No  matter  how  satisfactory  it  would  be  to  have  all 
biological  facts  interpreted  with  a  primer  simplicity, 
the  truth  is  that  animals  and  plants  are  complex  organ- 
izations. Probably  only  the  tiniest  fraction  of  the  germ 
cell  constitution  of  any  organism  has  ever  been  analyzed 
through  Mendelian  methods,  yet  in  the  pomace  fly 
Drosophila  melanog aster,  in  which  there  are  only  four 
pairs  of  chromosomes,  Morgan  and  his  associates  have 
traced  the  hereditary  transmission  of  well  over  one  hun- 
dred factors,  each  of  which  has  one  or  more  functions  to 
perform  in  the  development  of  characters  in  the  adult. 
It  is  obvious  that  with  such  a  large  number  of  characters 
and  such  a  small  number  of  chromosomes,  a  single  chro- 
mosome must  carry  many  factors.  This  conclusion 
granted,  it  would  seem  as  if  any  of  these  groups  of  fac- 
tors carried  by  a  single  chromosome  would  necessarily 
behave  as  single  factors ;  in  other  words,  they  would  enter 


62  INBREEDING  AND  OUTBREEDING 

a  cross  in  a  group  and  be  segregated  in  a  group  in  the 
F2  generation. 

Such  cases  have  appeared  in  breeding  experiments, 
but  they  are  very  rare  and  are  probably  not  what  they 
seem,  because  of  an  insufficient  number  of  individuals 
from  which  to  draw  conclusions.  What  usually  happens 
is  for  these  sets  of  factors  to  tend  to  hang  together  at  the 
reduction  division  in  the  F^  generation.  They  tend  to  be 
linked,  but  the  linkage  is  often  broken. 

An  example  from  Morgan's  work  on  the  pomace  fly 
will  make  this  clear.  If  a  female  fly  with  black  body  and 
vestigial  wings  be  crossed  with  a  wild  male  having  a  gray 
body  and  long  wings,  the  result  is  offspring  like  the  male, 
gray  body  and  long  wings  being  dominant.  Now,  since 
these  Fi  individuals  have  one  chromosome  containing  the 
factors  for  black  body  and  for  vestigial  wings,  and  a 
homologous  chromosome  containing  the  factors  for  gray 
body  and  for  long  wings,  one  would  expect  gametes  of 
only  two  kinds  to  be  formed  at  the  maturation  of  the  eggs 
and  sperms.  If  this  were  true,  an  F^  generation  ob- 
tained by  mating  a  male  and  a  female  from  the  F^  genera- 
tion should  consist  of  3  flies  having  long  wings  and 
gray  bodies  to  1  fly  having  vestigial  wings  and  a  black 
body.  But  this  is  not  the  result  obtained.  In  addition  to 
large  numbers  of  flies  of  this  type,  there  are  smaller  num- 
bers of  flies  characterized  by  long  wings  and  black  bodies, 
and  by  vestigial  wings  and  gray  bodies.  Such  a  result, 
on  a  chromosome  basis,  could  only  be  obtained  through  the 
homologous  chromosomes  interchanging  their  factors  at 
the  reduction  division. 

There  is  good  cytological  evidence  that  such  an  inter- 
change of  chromosome  parts  does  take  place  at  game- 


THE  MECHANISM  OF  HEREDITY 


63 


togenesis.  At  the  time  when  the  homologous  pairs  of 
chromosomes  approach  each  other  just  previous  to  the 
reduction  of  the  chromosome  number  to  half,  they  twist 
around  each  other.  Often  they  retain  their  individuality 
as  they  pass  to  the  daughter  cells;  but  sometimes  they 
break  at  various  places,  join  their  parts  in  a  different 
combination  and  pass  to  the  daughter  cells  in  their  new 
guise.    The  diagram  will  make  this  clear. 


A 

<-♦ 

a 

will  givd 

A 

and 

a 

B 

i-* 

b 

B 

b 

op 


A 

\   / 

a 

A 

a 

\y.\/             wiJI     A{„» 

and 

b 

X 

\ 

9 

b 

6 

FiQ.  20. — Diagram  to  illustrate  gamete  formation  in  a  dihybrid  in  linked   inheritance. 


The  easiest  way  to  determine  the  frequency  with  which 
these  breaks  in  linked  characters  occur,  and  a  way  which 
gives  it  in  terms  of  chromosome  crossovers  is  to  mate  Fi 
individuals  back  to  the  double  recessive  type.  When  the 
Fi  male  in  the  cross  just  described  is  mated  with  a  black 
vestigial  female,  only  two  classes  of  offspnng  are  pro- 
duced ;  half  are  black  vestigial  and  half  are  gray  long  in 
type.  The  F^  male  produces  only  two  kinds  of  gametes. 
There  is,  therefore,  no  crossing  over  between  the  chromo- 
somes of  the  male. 

On  the  other  hand,  when  the  F^  female  is  mated  with 
a  black  vestigial  male,  four  types  of  offspring  are 
produced : 

Non-crossovers  Crossovers 

Black  vestigial  Gray  long  Black  long  Gray  veslig-ial 

41.5  per  cent.  41.5  per  cent.      8.5  per  cent,  8.5  per  cent. 


64 


INBREEDINa  AND  OUTBREEDING 


^yn 


Gametes   of  Pj 


QZ^^ 


Individuals  of  Fj 
Gray  Long 


Gametes  of  Male 
All   direct   segregates 


C-ametes  of  Feinal( 
Direct    sigre^ates 


S^Z3  r^"^ 


Double  recessive 

blacVt   vestigial 

used    m   bacV<    crosses 


black 
Long 
50. OJ^ 


Gray 
vestigial 

so.oji 


black 

Lone 

41.5^ 


Gray 


black 


vestigial      vestigial 
41.5^  B,5% 


Gray 

Long 
8.5% 


FiQ.  21. — Diagram  to  illustrate  linkage  breaks  or  "crossing-over"  between  the  loci 
containing  the  factors  for  the  allelomorphic  pairs.  Gray  body,  black  body  and  long  wing — 
vestigial  wing  in  Drosophila.     (After  Morgan.) 

A  complete  visualization  of  this  series  of  matings  is 
given  in  Fig.  21. 


THE  MECHANISM  OF  HEREDITY  65 

The  peculiar  fact  tliat  there  is  no  crossing  over  in 
the  males  need  not  concern  us  here.  In  other  species 
there  is  evidence  of  crossing  over  in  both  sexes.  What  is 
important  is  that  crossing  over  does  occur  with  a  definite 
frequency  and  this  frequency  is  constant  for  any  par- 
ticular pair  of  characters  except  when  modified  in  various 
ways  which  can  be  given  concrete  explanations.  It  does 
not  matter,  moreover,  whether  the  two  factors  enter  the 
cross  as  ^'^  or  as  ^j^ ,  crossing  over  is  the  same  in  each 
case;  that  is,  the  tendency  is  just  as  great  for  Ah  to  stay 
together  after  they  are  in  that  combination  as  for  AB  to 
stay  together  when  that  particular  combination  charac- 
terizes the  individual. 

The  gross  result  of  several  thousand  experiments  of 
this  character,  therefore,  is  to  associate  Mendelian  in- 
heritance more  definitely  than  ever  with  the  chromosomes. 
What  seemed  to  be  an  exception,  furnished  striking  evi- 
dence in  support  of  the  theory.  It  is  true,  a  few  isolated 
instances  of  characters  which  appear  to  be  carried  by 
cell  substances  other  than  the  chromosomes  have  been 
discovered ;  but  it  is  pretty  clear  that  in  the  main  all  of  the 
varied  characters  which  differentiate  the  individuals 
within  a  single  species — and  these  are  the  only  ones 
which  can  be  studied  profitably  through  crosses — are  con- 
trolled by  the  distribution  of  factor  units  which  lie 
within  the  chromosomes. 

We  can  visualize  the  whole  process  of  heredity  by 
means  of  chromosome  diagrams  just  as  we  can  visualize 
the  whole  process  of  chemical  recombination  through 
models  of  the  atoms  constructed  to  fit  the  facts  furnished 
by  chemical  reactions.  The  result  is  that  we  can  do  much 
toward  predicting  what  will  happen  under  given  con- 

5 


66  INBEEEDING  AND  OUTBREEDING 

ditions  because  we  know  wliat  has  happened  under  sim-  !> 
ilar  conditions.  For  example,  it  having  been  determined 
that  in  the  pomace  fly;  many  characters  are  linked  to  chro- 
mosomes whose  distribution  parallels  that  of  sex,  we 
know  it  to  be  much  more  than  a  guess  to  say  that  the 
color-blindness  of  man  of  which  the  hereditary  distribu- 
tion was  described  in  Chapter  III,  is  controlled  by  a  factor 
lying  in  the  sex  chromosome  and  recessive  to  the  normal. 
Though  the  whole  mechanism  in  the  higher  plants  and 
animals  can  thus  be  pictured  as  one  of  sexual  reproduc- 
tion, in  its  details  the  results  are  still  too  complex  to 
analyze  as  concretely  as  the  cases  given  for  illustration. 
Several  thousand  concrete  differences  between  plants  of 
the  various  angiosperm  families  and  between  animals  in 
at  least  three  different  phyla  have  been  followed  through 
pedigree  cultures  sufficiently  carefully  to  make  possible 
a  definite  factorial  analysis  of  their  hereditary  transmis- 
sion. This  has  been  possible,  first,  because  variation  has 
taken  place  in  these  factors,  enabling  one  to  follow  the 
transmission  of  each  member  of  an  allelomorphic  pair, 
and,  second,  because  this  variation  has  been  somewhat 
qualitative  in  nature.  Unfortunately  for  the  peace  of 
mind  of  the  biologist,  however,  the  more  numerous  differ- 
ences between  animals  and  between  plants  are  the  quanti- 
tative differences,  the  variations  which  make  organs  a 
little  larger  or  a  little  smaller.  Now  it  is  a  great  deal 
easier  to  determine  the  transmission  of  the  factor  differ- 
ences which  determine  that  one  flower  shall  be  red  and 
another  white  than  it  is  to  trace  the  distribution  of  the 
factors  which  determine  that  one  flower  shall  be  one  inch 
and  another  two  inches  long.  Nevertheless,  through  the 
efforts  of  numerous  investigators  it  has  been  possible  to 


THE  MECHANISM  OF  HEREDITY  67 

show  that  such  hereditary  differences  behave  as  should 
be  expected  if  their  inheritance  follows  the  same  laws 
as  do  the  simpler  characters.  The  basis,  as  one  might 
say,  of  the  Mendelian  interpretation  of  size  differences  is 
the  proof  that  practically  aU  qualitative  characters  are 
affected  by  numerous  factors.  Sometimes  there  are  two 
or  more  factors  which  produce  nearly  identical  visible 
results,  but  more  often  the  character  complex  is  affected 
in  different  ways  and  in  various  degrees  by  particular 
factors.  Whether  the  character  develops  at  all  or  not 
seems  to  be  due  to  the  presence  or  absence  of  one  or  more 
main  factors,  but  given  the  presence  of  these  factors  the 
degree  of  development  may  be  influenced  by  many  sub- 
sidiary factors  or  modifiers.  Now  these  modifiers  being 
transmitted  independently  of  one  another  and  of  the  prin- 
cipal factor  or  factors,  an  individual  carrying  certain 
modifiers  and  lacking  the  principal  factor  may  be  crossed 
with  an  individual  carrying  the  main  factor  and  lacking 
the  modifiers.  The  result  is  a  series  of  recombinations 
among  the  germ  cells  of  the  F^  generation  which  produces 
F2  individuals  carrying  various  groups  of  modifiers  and 
therefore  developing  the  character  complex  under  con- 
sideration in  different  degrees. 

If  one  studies  carefully  such  crosses  as  the  one  just 
described,  he  finds  that  a  number  of  general  conditions 
are  fulfilled. 

1.  Wlien  pure  or  homozygous  races  are  crossed,  the 
jPi  populations  are  similar  to  the  parental  races  in  uni- 
formity. This  conclusion  devolves  from  observations 
that  if  any  particular  factors  AA  and  an  are  homozygous 
in  the  parental  races,  they  can  only  form  Aa  individuals 
in  the  Fj  generation. 


68  INBEEEDING  AND  OUTBREEDING 

2.  If  the  parental  races  are  pure,  F2  populations  are 
similar,  no  matter  what  F^  individuals  produce  them, 
since  all  variability  in  the  F^  generation  is  the  result  of 
varying  external  conditions. 

3.  The  variability  of  the  F2  populations  produced 
from  such  crosses  should  be  much  greater  than  that  of  the 
JPi  populations,  and  if  a  sufficient  number  of  individuals 
are  produced  the  grandparental  types  should  be  recov- 
ered. The  fulfillment  of  this  condition  comes  about  from 
the  general  laws  of  segregation  of  factors  in  F^  and  their 
recombination  in  F2. 

4.  In  certain  cases  F^  individuals  should  be  produced 
sho^ving  a  greater  or  a  less  extreme  development  of  the 
character  complex  than  either  grandparent.  This  is 
merely  the  result  of  recombination  of  modifiers,  as  was 
explained  above. 

5.  Individuals  of  different  types  from  the  F2  genera- 
tion should  produce  populations  differing  in  type.  The 
idea  on  which  this  statement  is  based  is,  of  course,  that  all 
F2  individuals  are  not  alike  in  their  inherited  constitution 
and  therefore  must  breed  differently. 

6.  Individuals  either  of  the  same  or  of  different  types 
chosen  from  the  F2  generation  should  give  F^  populations 
differing  in  the  amount  of  their  variability.  This  con- 
clusion depends  on  the  fact  that  some  individuals  in  the 
F2  generation  will  be  heterozygous  for  many  factors  and 
some  heterozygous  for  only  a  few  factors. 

Such  are  the  conditions  which  must  be  fulfilled  by 
crosses  exhibiting  size  differences  if  we  are  to  visualize 
their  inheritance  in  the  same  way  as  we  visualize  the  in- 
heritance of  qualitative  characters  such  as  color.  If  the 
size  differences  are  controlled  by  numerous  germ-cell  f  ac- 


THE  MECHANISM  OF  HEEEDITY  69 

tors,  the  distribution  of  the  latter  cannot  be  followed  with 
the  same  ease  as  one  would  follow  the  distribution  of 
cotyledon  colors  in  the  garden  pea.  This  is  time  because 
the  visible  effects  of  certain  factors  is  sure  to  be  very 
small,  and  because  varying  external  conditions  obscure 
the  effects  of  inheritance.  For  example,  a  plant  which 
through  its  inheritance  should  become  6  feet  tall  under 
average  conditions  may  become  only  4  feet  tall  if  planted 
in  a  sterile  soil,  but  a  plant  which  under  average  condi- 
tions would  become  only  4  feet  tall  might  become  5  feet 
tall  if  grown  in  a  very  fertile  soil. 

Nevertheless,  in  spite  of  these  drawbacks,  one  can 
select  size  characters  for  study  which  are  influenced  but 
slightly  by  external  conditions,  and  by  studying  large 
numbers  through  several  generations,  and  by  applying 
mathematical  tests  to  determine  the  uniformity  or  the 
variability  of  the  resulting  populations,  he  can  find  out 
whether  quantitative  characters  satisfy  the  six  require- 
ments seen  to  be  fulfilled  by  qualitative  characters.  This 
has  been  done  in  numerous  cases,  and  the  results  firmly 
convince  all  unprejudiced  investigators  that  the  inheri- 
tance of  all  types  of  characters  is  the  same. 

Table  I,  from  crosses  between  two  varieties  of  Nico- 
tiana  longiflora  ^^  differing  in  the  size  of  their  flowers, 
illustrates  the  point.  One  does  not  need  any  refined 
mathematical  methods  to  see  that  when  the  small  variety 
having  flowers  about  40  mm.  in  length  is  crossed  ^^dth  the 
large  variety  having  flowers  about  94  mm.  in  length;  the 
result  is  a  uniform  F^  population  having  flowers  about  64 
mm.  in  length.  The  two  Fn  populations  which  it  produced 
are  much  more  variable ;  and  one  can  easily  calculate  that 
if  several  thousand  plants  had  been  gro^^^l  instead  of 


70 


INBEEEDING  AJ^D  OUTBREEDING 


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vf 


THE  MECHANISM  OF  HEREDITY  71 

about  200,  the  grandparental  sizes  probably  would  have 
been  obtained.  Furthermore,  if  one  studies  the  results 
obtained  in  the  F^,  F^  and  F^  generations,  considering 
only  the  range  of  their  variability,  it  is  dear  they  differ  in 
both  type  and  extent  of  variation. 

No  assumptions  unproved  for  the  inheritance  of  quali- 
tative characters  are  necessary  for  thus  visualizing  the 
inheritance  of  quantitative  characters,  and  no  facts  dis- 
covered in  tracing  the  inheritance  of  other  characters — 
such  as  those  involving  linkage — are  overlooked.  But  in 
order  to  picture  the  situation  easily,  let  us  assume  that 
dominance  is  usually  absent  (often  the  case),  that  two 
doses  (i.e.,  the  homozygous  condition)  of  a  factor  have 
twice  the  effect  of  one  dose  (true  for  all  practical  pur- 
poses), that  independent  factors  cumulative  in  their  oper- 
ation are  allelomorphic  to  their  absence  in  the  hybrid 
(linkage  though  it  complicates  matters,  does  not  change 
our  reasoning). 

Let  us  assume  a  case  of  ^* blended''  inheritance  where 
all  fluctuations  due  to  environment  are  eliminated.  A 
plant  12  inches  tall  is  supposed  to  be  crossed  with  a  plant 
28  inches  tall.  The  difference  between  them  is  16  inches. 
If  this  difference  is  due  to  one  allelomorphic  pair  in  which 
dominance  is  absent,  the  F^  generation  is  all  intermediate 
— about  20  inches — and  the  JPg  generation  falls  into  three 
classes  in  which  two  represent  the  grandparental  forms 
and  one  represents  the  F^  form.  Twenty-five  per  cent, 
are  12  inches  tall,  fifty  per  cent,  are  20  inches  tall  and 
twenty-five  per  cent,  are  28  inches  taU. 

But  suppose  this  16-inch  difference  between  the 
parents  is  represented  by  two  allelomorphic  pairs  instead 
of  one.    The  F^  generation  is  again  20  inches  tall,  but 


72 


INBEEEDING  AND  OUTBREEDING 


instead  of  there  being  three  classes  in  F2,  there  are  five 
classes,  viz.,  12,  16,  20,  24  and  28  inches,  and  they  appear 
in  the  ratio  1:4:6:4:1.  Each  grandparental  type 
appears  once  out  of  sixteen  times. 

The  way  this  ratio  is  obtained  is  by  simple  recombina- 
tion, but  as  dominance  is  absent,  each  time  a  single 
** presence''  factor  is  added,  the  height  is  increased  four 
inches. 


9 


1  AABB 

2  AaBB 
2  AABh 
4  AaBh 

1  AAhh 

2  Aahh 

1  aaBB 

2  aaBh 

1  aahh 


28  inclies 

24  inches 

24  inclies 

20  inches 

20  inches 

16  inches 

20  inches 

16  inches 

12  inches 


If  three  independent  size  characters  instead  of  two 
were  involved  in  this  cross,  the  F^  individuals  would  fall 
in  the  same  class  as  before,  but  the  Fn  classes  would  be 
seven  in  number  and  the  grandparental  sizes  would  each 
be  recovered  only  once  out  of  sixty-four  times.  For  four 
factors  there  would  be  nine  classes  of  F^  individuals,  and 
the  grandparental  t^^oes  would  each  occur  only  once  out 
of  two  hundred  and  fifty-six  times;  while  with  only 
eight  factors,  the  forms  of  the  grandparents  would  each 
appear  only  once  out  of  65,536  times,  and  it  would  be 
quite  remarkable  if  they  were  ever  recovered  from  an 
ordinary  cross. 

The  entire  scheme  of  this  type  of  inheritance  can  be 
expressed  in  mathematical  form  just  like  ordinary  Men- 
delian  inheritance  with  full  dominance.    Let  us  recall  that 


THE  MECHANISM  OF  HEREDITY  73 

the  F2  Mendelian  expression  for  N  allelomorphic  pairs 
when  dominance  is  complete  is  the  expanded  bionominal: 

(3  +  l)»  or  (3/4  4-1/4)** 
N==l     (3  +  1)1  =  34-1 

iV  =  2     (3 +  1)2  =  32  4-34-34-1=- 94- 34-3  +  1 
iV  =  3     (3  +  1)3  =  33  +  3(32)  2+3(3)41  =  27  +  9  +  9  +  9 
+  3  +  3+3  +  1 

Likewise,  the  expanded  bionomial  (V2  +  %)^"  gives  the 
numerical  relationships  when  dominance  is  absent  and  N 
represents  the  number  of  allelomorphic  pairs.  The  ex- 
pression is  (%  +  i/2)^"  instead  of  {1/2  +  1/2)"  because  it  is 
supposed  that  the  presence  of  any  allelomorphic  pair  in 
the  heterozygous  condition  produces  one-half  the  visible 
effect  on  the  character  that  is  produced  when  the  heredi- 
tary factors  are  present  in  the  homozygous  condition. 
When  N  is  very  large  the  frequencies  with  which  the  dif- 
ferent classes  occur  form  a  regular  curve  called  the  nor- 
mal curve  of  error.  This  is  the  curve  that  is  produced 
when  the  errors  in  any  physical  measurement  are  sim- 
ilarly plotted,  using  as  classes  any  constant  deviation 
from  the  average,  as  a^  2a,  3a,  etc.  This  same  curve  is 
also  produced  when  one  plots  the  fluctuations  of  many 
organic  characters  produced  by  the  infinite  complexity  of 
external  conditions. 

If  no  non-heritable  fluctuations  intervened  to  obscure 
the  class  to  which  any  particular  zygote  belongs,  there- 
fore, one  should  expect  the  following  classes  in  Fn  when 
parents  of  different  sizes  differing  in  N  allelomorphic 
pairs  are  crossed.  The  extremes  represent  the  grand- 
parental  types  in  each  case,  and  the  intermediate  classes 
theoretically  divide  the  difference  between  the  parents 
into  aliquot  parts.    It  should  be  noted,  however,  that  this 


74  INBEBEDING  AND  OUTBREEDING 

is  theory  only ;  in  reality  the  influence  of  one  factor  may 
be  somewhat  different  from  that  of  another  factor. 

N=l  12   1 

iV==2  14   6   4   1 

N  =  3  1   6  15  20  15   6   1 

Ar  =  4       1   8  28  56  70  56  28   8   1 

N  =  5  1  10  45  120  210  252  210  120  45  10   1 

Let  us  now  note  a  few  of  the  practical  difficulties  in 
interpreting  results  that  may  follow  this  method  of  in- 
heritance. In  the  theoretical  example  that  we  have  used 
for  the  sake  of  clearness,  it  was  assumed  that  there  were 
no  non-heritable  fluctuations  due  to  environment.  Unfor- 
tunately this  is  not  the  case  in  nature.  Fluctuations  are 
everywhere  present.  They  would  obscure  the  classes  to 
which  individuals  belong  even  if  these  class  differences 
were  quite  large.  And  since  they  are  usually  small,  the 
change  of  individual  form  due  to  environmental  causes 
makes  it  impossible  to  separate  an  Fg  population  into  the 
true  classes  to  which  they  belong  gametically.  Nor  is  this 
the  whole  trouble.  If  the  table  showing  the  expected  re- 
sults with  two  pairs  of  size  characters  is  examined,  it  is 
found  that  not  all  the  individuals  that  belong  to  a  par- 
ticular size  class  have  the  same  zygotic  formula.  For  this 
reason  one  cannot  pick  out  zygotes  of  a  certain  size  and 
expect  them  to  breed  the  same.  Their  potentialities  are 
likely  to  be  different.  Furthermore,  practical  breeding 
results  are  undoubtedly  complicated  by  cases  of  correla- 
tion. This  correlation  need  not  be  gametic,  though  such 
cases  in  all  likelihood  do  occur;  it  may  be  merely  physi- 
ological. For  example,  a  maize  plant  might  have  the 
gametic  possibilities  of  small  plant  size  and  large  ear 
size,  but  it  would  be  foolish  to  expect  that  a  plant  capable 


THE  MECHANISM  OF  HEREDITY  75 

of  only  a  limited  amount  of  development  could  bear  as 
large  an  ear  as  if  it  were  as  a  whole  capable  of  greater  size 
development.  Thus  it  must  not  be  expected  that  theoreti- 
cal possibilities  are  always  expressed  perfectly  in  nature, 
any  more  than,  it  should  be  expected  that  theoretical 
piiysical  calculations  concerning  known  laws  should  agree 
perfectly  with  experimental  data.  Plants  and  animals  do 
indeed  seem  to  have  in  their  reproductive  cells  a  mosaic 
of  independently  transmissible  factors,  but  a  plant  or 
animal  is  certainly  not  to  be  described  as  a  mosaic  of  in- 
dependent unit  characters.  These  factors  that  appear  to 
be  independent  in  heredity  act  and  react  upon  one  another 
in  complex  ways  during  their  development. 

Hundreds  of  studies  on  quantitative  characters  have 
been  made.  They  all  have  the  same  result.  Mendelian 
inheritance  rules.  Plants  appear  to  be  less  complex  than 
animals.  A  size  complex  in  an  animal  seems  to  be  the 
result  of  the  interaction  of  a  large  number  of  factors,  a 
size  complex  in  a  plant  appears  to  be  the  result  of  the 
interaction  of  a  small  number  of  factors.  But  the  mode 
of  inheritance  is  always  the  same.  It  is  the  result  of  the 
behavior  of  the  chromosomes,  and  one  can  picture  it  with 
the  greatest  ease  with  the  simple  diagram  of  the  reduc- 
tion division  at  gametogenesis  if  he  fancies  to  himself  that 
the  chromosomes  are  carrying  bodies  for  the  unit  factors 
of  heredity,  that  the  arrangements  within  them  are  a  near 
approach  to  perfection,  that  exchanges  of  contents  may 
be  made  only  with  regularity  and  precision  so  no  essen- 
tial feature  of  the  mechanism  shall  break  down. 

In  thus  visualizing  the  process  of  heredity,  one  must 
not  be  so  overcome  by  the  beauty  of  the  picture  that  he  is 
unable  to  realize  just  what  has  been  done.    He  must  not 


76  INBEEEDING  AND  OUTBREEDING 

forget  which  part  of  this  diagrammatic  representation 
of  the  heredity  mechanism  is  fact  and  which  part  is 
theory,  for  confusion  between  the  two  has  led  to  a  regret- 
table controversy  over  a  point  which  is  of  paramount  im- 
portance in  any  discussion  of  inbreeding  and  outbreeding 
— the  stability  of  inherited  factors. 

The  relation  between  fact  and  theory  in  the  Mendelian 
conception  of  inheritance  is  this:  Various  kinds  of  ani- 
mals and  of  plants  were  crossed  and  the  results  recorded. 
With  the  repetition  of  experiments  under  comparatively 
constant  environments  these  results  recurred  with  suf- 
ficient regularity  to  justify  the  use  of  a  notation  in  which 
theoretical  factors  or  genes  located  in  the  germ  cells  re- 
placed the  actual  somatic  characters  found  by  experiment. 
Later,  the  observed  behavior  of  the  chromosomes  justi- 
fied localizing  these  factors  as  more  or  less  definite  physi- 
cal entities  residing  in  them.  Now  the  data  from  the 
breeding  pen  or  the  pedigree  culture  plot  and  the  ob- 
servations on  the  behavior  of  the  chromosomes  during 
gametogenesis  and  fertilization  are  facts.  The  factors 
are  part  of  a  conceptual  notation  invented  for  simplifying 
the  description  of  the  breeding  facts  in  order  to  utilize 
them  for  purposes  of  prediction,  just  as  the  chemical 
atom  is  a  conception  invented  for  the  purpose  of  simpli- 
fying and  making  useful  observed  chemical  phenomena. 
As  used  mathematically,  both  the  genetical  factor  and 
the  chemical  atom  are  concepts,  but  biological  data  lead 
us  to  believe  that  the  term  factor  represents  a  biological 
reality  of  whose  nature  we  are  ignorant,  just  as  a  molecu- 
lar formula  represents  a  physical  reality  of  a  nature  yet 
but  partly  known. 

With  this  distinction  in  mind,  one  may  treat  the  fac- 


THE  MECHANISM  OF  HEREDITY  77 

tor — or  the  atom — from  two  points  of  view,  either  as  a 
mathematical  concept  or  a  physical  reality.  As  a  mathe- 
matical concept  it  is  the  unit  of  heredity,  and  a  unit  in 
any  notation  must  be  stable.  If  one  creates  a  hypothetical 
unit  by  which  to  describe  phenomena  and  this  unit  varies, 
there  is  really  no  basis  for  description.  He  is  forced 
to  hypothecate  a  second  fixed  unit  to  aid  in  describing 
the  first. 

The  point  at  issue  in  this  connection  may  be  explained 
as  follows :  Characters  do  vary  from  generation  to  gen- 
eration, and  the  question  to  be  decided  is,  how  much  of 
this  variation  is  due  to  the  recombination  of  factors  (con- 
sidered now  as  physical  entities)  and  how  much  is  due  to 
change  in  the  constitution  of  the  factors  themselves.  The 
obvious  way  to  determine  such  a  matter  is  first  to  appeal 
to  Nature  and  see  whether  it  is  possible  for  characters 
to  have  a  long*  period  of  stability  under  any  conditions ; 
and,  second,  to  investigate  the  stability  of  characters 
when  the  environment  is  comparativeily  constant  and 
change  due  simply  to  recombinations  of  heterozygous 
factors  is  eliminated. 

Of  the  results  of  the  appeal  to  Nature  only  one  need 
be  mentioned.  Wlieeler  ^^^  has  found  that  ants  preserved 
in  amber  of  the  Oligocene  period,  fossils  which  are  better 
preserved  than  any  others,  and  which  are  thought  to  be  at 
least  3,000,000  years  old,  are  practically  identical  with 
living  species.  The  only  points  of  variance  to  be  observed 
are  slight  differences  in  shade  of  color,  something  prob- 
ably due  to  the  mode  of  preservation.  Thus  it  is  clear  that 
organic  characters  may  remain  stable  for  periods  of  time 
so  great  as  to  be  beyond  our  powers  of  realization. 

Investigations  as  to  the  effect  of  selection  on  homo- 


78  INBREEDING  AND  OUTBREEDING 

zygous  hermaphroditic  plants  which  are  self-fertilized 
naturally — the  only  material  having  a  critical  value  with 
this  mode  of  attack,  if  we  except  unicellular  organisms 
reproducing  asexually — have  been  made  by  a  number  of 
biologists  following  the  lead  of  Johannsen/^®  who  opened 
up  the  possibilities  of  this  type  of  experiment.  The  re- 
sults have  always  been  the  same.  Characters  are  remark- 
ably stable.  They  do  change,  but  they  change  so  rarely 
that  a  more  useful  purpose  is  served  by  identifying  the 
physical  unit  factor  with  the  mathematical  factor  unit, 
than  to  assume  without  justification  that  the  physical 
factor  is  constantly  changing  and  must  be  described  by 
complex  mathematical  formulae  using  other  hypothetical 
units  having  no  warrant  for  a  physical  existence.  It  is 
true,  two  investigations  by  Jennings  ^^"^  and  by  Middle- 
ton  ^*2  have  shown  a  seemingly  more  unstable  condition  in 
the  infusorians  Difflugm  coronata  and  StylonycMa  pustu- 
lata.  But  there  are  several  reasons  for  not  believing  con- 
clusions derived  from  data  on  these  animals  applicable  to 
the  higher  plants  and  animals  in  which  our  real  interest 
lies,  without  mentioning  several  technical  points  which 
might  lead  to  interpretations  different  from  those  given 
by  the  authors.  First,  they  are  cases  of  asexual,  not  sex- 
ual, reproduction.  Second,  the  germ  plasm  of  the  infusoria 
may  not  be  insulated  from  the  effects  of  environment  as  is 
the  germ  plasm  of  the  higher  organisms.  Third,  measur- 
able differentiation  in  these  experiments  sometimes  took 
such  a  number  of  generations  that  in  man  it  would  take 
some  3000  years  to  produce  like  results. 

For  these  and  other  reasons  which  might  be  given, 
could  further  space  be  devoted  to  the  subject,  we  believe 
there  should  be  no  hesitation  in  identifying  the  hypotheti- 


THE  MECHANISM  OF  HEREDITY  79 

cal  factor  unit  with  the  physical  unit  factor  of  the  germ 
cells.  Occasional  changes  in  the  constitution  of  these 
factors,  changes  which  may  have  great  or  small  elfects  on 
the  characters  of  the  organism,  do  occur;  but  their  fre- 
quency is  not  such  as  to  make  necessaiy  any  change  in 
our  theory  of  the  factor  as  a  permanent  entity.  In  this 
conception  biology  is  on  a  par  with  chemistry,  for  the 
practical  usefulness  of  the  conception  of  stability  in  the 
atom  is  not  affected  by  the  knowledge  that  the  atoms  of 
at  least  one  element,  radium,  are  breaking  down  rapidly 
enough  to  make  measurement  of  the  process  possible. 


CHAPTER  V 

MATHEMATICAL  CONSIDEEATIONS  OF 

INBREEDING 

The  term  inbreeding  can  be  used  in  a  relative  sense 
only,  except  when  dealing  with  hermaphroditic  organ- 
isms. To  say  that  one  individual  of  a  bisexual  species  is 
inbred  and  another  not  is  as  indefinite  as  saying  one  is 
short,  the  other  tall.  Strictly  speaking,  inbreeding  refers 
only  to  the  way  in  which  individuals  are  mated  together. 
This  fact  is  well  expressed  by  Pearl,^^^  who  says:  ''  It  is 
clear  that  underlying  all  definitions  of  inbreeding  is  to  be 
found  the  concept  of  a  narrowing  of  the  network  of 
descent  as  a  result  of  mating  together  at  some  point  in 
the  network  of  individuals  genetically  related  to  one  an- 
other in  some  degree.  Let  us  take  this  as  our  basic 
concept  of  inbreeding.  It  means  that  the  number  of  po- 
tentially different  germ-to-germ  lines  or  *^ blood-lines" 
concentrated  in  a  given  individual  is  fewer  if  the  individ- 
ual is  inbred  than  if  he  is  not.  In  other  words,  the  inbred 
individual  possesses  fewer  different  ancestors  in  some 
particular  generation  or  generations  than  the  maximum 
possible  number  for  that  generation  or  generations.^* 
Thus,  according  to  the  evolutionist's  conception  of  the 
origin  of  species  by  natural  selection,  not  only  are  all 
members  of  a  species  related  in  some  degree,  however 
remote,  but  all  members  of  all  species  from  any  one 
original  life-sparh  presumably  are  members  of  one  inbred 
liyie.  This  wholly  ridiculous  conclusion  follows,  because 
the  lines  of  descent  terminating  in  any  one  individual, 
though  they  radiate  back  in  widening  angles  for  a  time, 

80 


MATHEMATICAL  CONSIDERATIONS         81 

would  be  seen  to  gather  together  again  in  a  comparatively 
few  individuals  if  the  pedigree  of  the  species  could  be 
traced  in  its  entirety. 

Such  a  reductio  ad  ahsurdum  is  not  altogether  value- 
less. It  shows  how  essential  it  is  for  one  to  recognize  the 
unavoidable  limitations,  the  desirability  of  definite  analy- 
sis, the  necessity  of  precise  methods  of  attack,  in  any 
consideration  of  the  proposition  he  may  undertake. 

There  are  three  distinct  phases  of  the  inbreeding  prob- 
lem, as  Pearl  has  pointed  out : 

1.  The  system  of  mating  with  regard  to  the  relation 
of  the  actual  number  of  ancestors  making  up  the  pedigree 
of  an  individual  to  the  total  possible  number. 

2.  The  constitution  of  each  individual  with  respect  to 
Mendelian  unit  factors  which  results  from  the  continued 
operation  of  a  given  system  of  mating  which  is  inbreeding. 

3.  The  physiological  effect  produced  upon  the  indi- 
vidual by  the  constitution  derived  from  this  system 
of  mating. 

The  first  two  phases  of  the  problem  are  capable  of 
abstract  mathematical  treatment.  The  third  can  be 
solved  only  by  experimental  investigation. 

Precise  methods  of  measuring  and  comparing  sj^stems 
of  mating  have  been  devised  by  Pearl  by  the  use  of  a 
Coefficient  of  Inbreeding  and  a  Coefficient  of  Relation- 
ship."   The  first  is  a  measure  of  the  actual  number  of 

o  Pearl  has  made  a  somewhat  more  precise  analysis  of  tlie  Inbrecdinf]j 
and  Relationship  Coefficients  in  later  papers,  175,  17G,  and  has  siiijj?ostc<i 
a  Partial  Inbreeding  Index,  in  a  percentage  which  one-half  of  the  ReUition- 
ship  Coefficient  is  of  the  Inbreeding  Coefficient.  This  constant  is  a  moaaure 
of  the  amount  of  inbreeding  due  to  relationship  between  the  sire  and  dam. 
Further,  he  has  described  a  single  numerical  measure  of  inbrwtling  for 
bisexual  organisms,  in  the  ratio  of  the  area  of  the  inbreetling  curve  in  any 
pedigree  to  the  area  of  the  maximum  (brother  X  sister)  curve. 

For  our  purposes,  it  is  unnecessary  to  consider  these  ext4?n8ion9  of 
Pearl's  studies  in  detail,  though  technically  they  are  very  valuable. 

6 


82  INBEEEDINa  AND  OUTBREEDING 

ancestors  compared  with  the  possible  nimiber.  It  is 
derived  from  the  formula: 

lOO/p         -q       \ 
\  n  +  1         n  +  1/ 

Z   ^    

n  p 

«  +  l 

where  pn^^  denotes  the  maximum  possible  number  of 
different  individuals  involved  in  the  matings  of  the  n  +  1 
generation,  and  gn+i  the  actual  number  of  different  indi- 
viduals involved  in  these  matings.  As  an  illustration,  any 
individual  in  bisexual  matings  has  two  parents  in  the  first 
ancestral  generation,  four  grandparents  in  the  second 
ancestral  generation,  and  so  on,  according  to  the  following 
symbolical  representation 

X -^->  (1)  2<->  (2)  4-^— >  (3)  8<— >(4)  16-<— >  (5)  32<-^  (»)2'» . . ., 

in  which  the  enclosed  numbers  represent  the  ancestral 
generations  (1  =  parents,  2  =  grandparents,  3  =  great- 
grandparents,  etc.),  and  the  other  figures  the  number  of 
ancestors.  In  the  second  or  earlier  generations  the  an- 
cestors may  not  all  be  different  individuals,  so  that  in  any 
generation  previous  to  the  parental  the  actual  number  of 
ancestors  may  be  less  than  the  possible  number.  For  ex- 
ample, in  brother  and  sister  mating,  any  individual  in- 
stead of  having  four  different  grandparents,  has  only  two. 
Expressed  symbolically,  as  above,  the  representation  for 
this  type  of  mating  would  be 

X  <-^  (1)2 -^->  (2)4-2/1  -^->  (3)8-7/2 <->  (4)  16-1/3  <->  (5)32-2/4. . . , 

where  2/1  =  2,  1/0  =  6,  y.^  =  14,  2/4  =  30. 

In  this  case  y  has  the  value  of  2"  -  2,  and  this  is  the  highest 

value  it  can  have  in  any  system  of  mating  where  two  indi- 


MATHEMATICAL  CONSIDERATIONS         83 

viduals  are  necessary  for  reproduction.  Applj^ng  the 
formula  given  above,  the  Coefficients  of  Inbreeding  for 
each  generation  in  brother  and  sister  mating  are : 

Zo  =  100     (2-2)=   0 

2 
Z^  =  100     (4-2)  =50 

4 

^2  =  100      (8-2)  =75 

8 
Z3  =  100   (16-2)  =  87.5 

16 

The  figures  obtained  are  the  differences  between  the 
possible  number  of  ancestors  and  the  actual  number  ex- 
pressed as  percentages  of  the  former.  By  plotting  these 
percentages  for  successive  generations  on  the  generation 
number  as  a  base,  a  curve  of  inbreeding  is  obtained  which 
can  be  compared  to  the  curves  obtained  by  other  systems 
of  matings.  This  comparison  is  shown  in  Fig.  22  for  the 
common  types  of  matings  as  worked  out  by  Pearl. 

From  these  curves  it  is  evident  that  continued  brother 
by  sister  and  double  first-cousin  matings  have  the  same 
effect,  although  the  latter  is  one  generation  behind  the 
former.  Also  the  curves  for  parent  by  offspring  and  sin- 
gle first-cousin  matings  are  similar  in  type,  but  show  the 
same  differences  in  position.  In  any  case  the  concentra- 
tion of  the  lines  of  descent  in  these  systems  of  inbreeding 
is  rapid,  until  after  fifteen  generations  no  individual  can 
have  more  than  a  fraction  of  one  per  cent,  of  the  number 
of  ancestors  theoretically  possible. 

The  Coefficient  of  Inbreeding  alone  tells  us  nothing 
as  to  the  relation  between  the  different  lines  of  descent. 


84 


INBEEEDINa  AND  OUTBEEEDING 


Two  individuals  may  have  the  same  Coefficients  of  In- 
breeding when  considered  for  any  given  number  of  gen- 
erations, but  differ  greatly  in  germinal  constitution.  This 
is  due  to  the  fact  that  the  two  lines  brought  together  in 
the  immediate  production  of  any  individual  may  or  may 
not  be  related.  For  example,  a  closely  inbred  animal  of 
one  breed  may  be  mated  to  another  closely  inbred  animal 


100 


80 


£   60 
a 


o 
I    40 


20 


. 

/^ 

5? 

^  1 

/ 

t 

1 

r 

f     t 
1    1 

1/ 

If 

1/ 
1/ 

1 

6  8  10 

Generations 


12 


14 


FiQ.  22. — Curves  of  inbreeding  showing  (o)  the  limiting  case  of  continued  brother 
X  sister  breeding,  wherein  the  successive  coefficients  of  inbreeding  have  the  maximum 
values;  (b)  continued  parent  offspring  mating;  (c)  continued  first  cousin  X  first  cousin 
mating  where  the  cousinship  is  double  (C^  XC^),  and  (d)  continued  first  cousin  X  first 
cousin  mating  where  the  cousinship  is  single  (C*  XC^).  The  continued  mating  of  uncle  X 
niece  gives  the  same  curve  as  C^  X  C^     (After  Pearl.) 

of  an  entirely  different  breed.  The  two  lines  of  descent 
would  then  be  totally  unrelated  as  far  as  the  known  pedi- 
grees are  concerned,  but  the  resulting  individual  would 
have  a  high  Coefficient  of  Inbreeding,  due  to  the  concen- 
tration of  ancestry  separately  in  the  two  ancestral  lines. 
To  give  some  measure  of  the  inter-relation  of  the  lines  of 
descent,  Pearl  has  devised  the  Coefficient  of  Eelationship, 
Ky  which  is  essentially  the  per  cent,  of  the  individuals  in 


MATHEMATICAL  CONSIDERATIONS         85 

each  of  the  descending  lines  which  are  also  represented 
in  the  other  line.  To  give  an  adequate  mathematical  esti- 
mation of  the  degree  of  inbreeding,  both  constants  are 
necessary.  There  is,  generally,  some  correlation  between 
them,  although  the  Coefficient  of  Relationship  may  be 
zero,  and  the  Coefficient  of  Inbreeding  still  be  high,  as  in 
the  illustration  just  given  in  which  the  progeny  comes 
from  a  pair  of  individuals  from  two  distinct  inbred  lines. 
The  application  of  these  methods  of  determining  the 
amount  of  inbreeding  is  illustrated  by  Pearl  from  the 
pedigrees  of  two  Jersey  bulls  as  follows : 

Inbreeding"  " Z"  and  Relationship  ''  (K)  ''  CoeOicients 

of 
King  Melia  Rioter  14th  and  Blossom's  Glorene 


^1 

Z,  {KJ 

0 

(0) 

0 

(0) 

A, 

Z,  (K,) 

25 

(0) 

0 

(0) 

^3 

Z.  {K,) 

25.00 

(50.00) 

12.50 

(0) 

^4 

Z\  (K,) 

37.50 

(G2.50) 

12.50 

(0) 

^5 

Z,  {K.J 

50.00 

(75.00) 

25.00 

(0) 

^6 

Z,  (K,) 

71.88 

(87.50) 

29.C9 

(0) 

^T 

Z,  (K,) 

81.25 

(92.19) 

35.94 

(0) 

^8 

Z,  {K,) 

90.63 

(92.97) 

40.23 

(0) 

The  method  of  making  the  calculations  is  explained 
clearly  and  concisely  by  the  originator  and  we  shall  not 
undertake  to  repeat  it  here.  What  we  are  interested 
in  is  the  genetic  meaning  of  the  figures  after  they  have 
been  obtained. 

The  Coefficient  of  Inbreeding,  Z,  has  to  do  solely  with 
total  relationship,  and  shows  the  intensity  of  inbreeding 
in  the  stockman's  sense  of  the  word  by  measuring  pre- 
cisely **the  proportionate  degree  to  which  the  actually 
existent  number  of  different  ancestral  individuals  fails  to 


86 


INBREEDING  AND  OUTBREEDING 


reach  the  possible  number,  and  by  specifying  the  location 
in  the  series  of  the  generation  under  discussion.  * '  King 
Melia  Rioter  14th  had  less  than  10  per  cent,  of  the  maxi- 
mum number  of  ancestors  in  the  7th  ancestral  generation, 
while  in  the  same  generation  Blossom's  Glorene  had  nearly 
60  per  cent.  From  these  figures  it  is  evident  that  King 
Meha  Rioter  14th  is  a  much  more  inbred  animal  than 


100 


80 


2   60 

a 

® 

m 

<S    40 


20 


/ 

^ 

:r:s:r=i 

1 
* 

r/ 

/ 

/ 

/ 

/ 

llA 

• 

f 

/ 

f 
1 

6  8 

Generations 


10 


12 


14 


^5'  Fig.  23. — Graphs  showing  (a)  the  total  inbreeding  (heavy  solid  line)  and  (b)  the  rela- 
tionship (heavy  broken  line)  curves  for  the  Jersey  Bull,  King  Melia  Rioter  14th.  The 
high  order  of  the  inbreeding  and  relationship  between  the  sire  and  dam  in  this  case  is  evident 
by  comparison  with  the  lighter  lines,  which  give  the  maximum  values  for  continued  brother 
X ^sister  and^parentlXioff spring  breeding. g    (AftertPearl.) 

Blossom's  Glorene.  A  clearer  demonstration  of  the  mat- 
ter, however,  is  found  in  Fig.  23,  where  the  curve  of  the 
total  inbreeding  of  King  Melia  Rioter  14th  plotted  from 
the  figures  just  cited  is  compared  with  the  curve  of  maxi- 
mum values  for  continued  brother  x  sister  and  parent  x 
offspring  matings. 

The  Coefficient  of  Relationship,  K,  might  better  be 
called  the  Coefficient  of  Cross-Relationship  to  distinguish 


MATHEMATICAX.  CONSIDERATIONS         87 

its  function  from  that  of  the  Coefficient  of  Inbreeding, 
since  it  is  a  measure  of  the  community  of  ancestry  of  the 
dam  and  the  sire. 

These  two  coefficients  taken  together,  then,  give  us 
the  first  quantitative  measure  of  inbreeding  as  a  system 
of  mating,  but  obviously  they  do  not  tell  anything  con- 
cerning the  actual  germinal  constitution  of  any  individual 
resulting  from  a  given  system  of  inbreeding.  This  fea- 
ture of  the  relationship  coefficients  is  nicely  illustrated 
by  one  of  PearPs  examples.  Clearly,  a  Holstein  cow  pro- 
duced by  continued  brother  x  sister  matings  (K  =:  100)  is 
very  different  in  its  germinal  constitution  from  a  cross- 
bred animal  obtained  by  mating  this  cow  vdih.  a  Jersey 
bull,  the  product  of  a  similar  system  of  inbreeding  (^  =  0) . 
Yet  the  Coefficients  of  Inbreeding  in  each  case  form  iden- 
tical series,  with  the  maximum  possible  value  of  Z  when 
K=0  one  generation  farther  removed  than  when  A"=100. 

Without  question  the  germinal  (or  may  one  call  it  the 
Mendelian?)  composition  of  any  individual  can  be  deter- 
mined only  by  actually  testing  its  breeding  qualities,  its 
transmissive  powers ;  and  the  effect  this  composition  may 
have  had  upon  its  development  can  be  measured  only  by 
comparison  with  other  individuals  of  known  genetic  con- 
stitution. But  an  indication  of  the  germinal  constitution 
of  an  individual  produced  by  any  long-continued  system 
of  inbreeding,  as  far  as  the  degree  of  heterozygosity  or 
homozygosity  is  concerned,  can  be  obtained  by  applyinc: 
the  laws  of  probability  to  Mendelian  formula?.  In  other 
words,  the  laws  of  probability  applied  to  Mendelian 
formulae  show  the  probable  homozygosity  or  heterozygos- 
ity of  the  generation  as  a  whole  for  any  number  of  Men- 
delian allelomorphic  pairs   with  any  given   system   of 


88  INBEElEDING  AND  OUTBREEDING 

inbreeding,  and  some  idea  of  the  composition  of  the  indi- 
vidual may  often  be  had  from  a  careful  consideration  of 
the  composition  of  the  generation  to  which  it  belongs. 

In  an  endeavor  to  demonstrate  the  effect  of  various 
systems  of  inbreeding  upon  Mendelian  constitution,  and  to 
appraise  the  effect  of  this  constitution  upon  develop- 
mental vigor,  let  us  approach  the  problem  from  the  op- 
posite direction. 

It  has  been  established  that  the  effect  produced  by 
crossing  depends  more  or  less  closely  upon  the  genetic 
diversity  of  the  types  which  produce  the  hybrid.  The 
usual  result  of  crossing  organisms  which  differ  in  many 
characters  is  a  first  generation  which  is  no  more  variable 
than  the  parental  types.  The  second  generation,  how- 
ever, may  be  expected  to  show  a  greater  variability  be- 
cause of  Mendelian  segregation.  The  amount  of  such 
variability  is  a  measure  of  the  diversity  of  the  parents 
which  produce  the  cross.  It  is  in  crosses  which  show 
greater  variability  in  the  second  generation  that  hybrid 
vigor  is  expected  in  the  generation  immediately  following 
the  cross.  When  such  hereditary  combinations  are  com- 
posed of  unlike  elements,  hybrid  vigor  is  commonly 
shown;  when  all  the  combinations  are  composed  of  like 
elements  hybrid  vigor  is  absent.  Hence,  in  crossed  species 
of  wild  or  domesticated  animals  and  plants  part  of  their 
vigor  may  be  the  result  of  dissimilar  hereditary  factors 
acting  together.  If  conditions  are  brought  about  by  which 
this  dissimilarity  in  aUelomorphic  combinations  is  re- 
duced or  lost  completely  a  partial  diminution  of  develop- 
mental energy  will  occur.  Since  there  is  a  constant 
tendency  for  inbreeding  of  whatever  kind  to  bring  about 
similarity    in    germinal    construction,    inbreeding    will, 


MATHEMATICAL  CONSIDERATIONS         89 

therefore,  frequently  cause  a  general  reduction  in  vigor. 
It  has  never  been  held  that  all  hereditary  factors 
are  equally  involved  in  this  effect  on  vigor.  Some 
are  considered  to  be  wholly  without  effect.  The  fact 
remains,  however,  that  the  increased  growth  and  extra 
vigor  commonly  resulting  from  hybridization  as  a  mass 
effect  is  intimately  associated  with  Mendclian  plie- 
nomena,  and  its  expression  is  roughly  proportional  to  the 
number  of  heterozygous  factors  present  and  disappears 
when  homozygosity  is  brought  about.  r.^ 

The  reduction  of  the  number  of  heterozygous  allele^ 
morphs  in  an  inbred  population  is  automatic  and  varies  , 
with  the  closeness  of  inbreeding.     In  self-fertilization  it  : 
follows  the  well-known  Mendelian  formula  by  which  any 
heterozygous  pair  forms  in  the  next  generation  50  per 
cent,  homozygotes  and  50  per  cent,  heterozygotcs  in  re- 
spect to  that  pair.     Since  the  homozygous  allelomorphs 
must  always  remain  constant  and  the  number  of  hetero- 
zygous factor  combinations  is  halved  each  generation  and 
one-half  added  to  the  homozygous  class,  the  reduction  in 
the  number  of  heterozygous  elements  proceeds  as  a  vari- 
able approaching  a  limit  by  one-half  the  difference  in 
each  generation.    The  curve  illustrating  this  approach  to 
complete  homozygosity  is  shown  as  No.  1  in  Fig.  24. 

As  lEast  and  Hayes  ^^  have  said:  ^^ Mendel,  in  his 
original  paper,  showed  that  if  equal  fertility  of  all  plants 
in  all  generations  is  assumed,  and,  furthermore,  if  every 
plant  is  always  self-fertilized,  then  in  the  nth  generation 
the  ratio  of  any  particular  allelomorphic  pair  {A,  a)  would 
be  2"  -  lAA  :  2Aa:  2"  -  laa.  If  we  consider  only  homo- 
zygotes and  heterozygotcs,  the  ratio  is  2"  -1 : 1.  Of 
course,  the  matter  is  not  quite  so  simple  when  several 


90 


INBEEEDING  AND  OUTBREEDING 


allelomorplis  are  concerned,  but  in  the  end  the  result  is 
similar.    Heterozygotes  are  eliminated  and  homozygotes 


100% 


Percent,  of  Heterozygous 
Individuals  in  Each  Selfed 
Generation  when  the  Number 
of  Allelomorphs  Concerned 
Are:  1,5,10,15. 


Segregating  Generations 

Fio.  24. — Graphs  showing  the  reduction  of  heterozygous  individuals  and  of  heterozygous 
allelomorphic  pairs  in  successive  generations  of  self-fertilization. 

remain.  The  probable  number  of  homozygotes  and  any 
particular  class  of  heterozygotes  in  any  generation  r  is 
found  by  expanding  the  binomial  1  +  (2''  - 1)"  where  n  rep- 


MATHEMATICAL  CONSIDERATIONS         91 

resents  the  number  of  character  pairs  involved.  The 
exponent  of  the  first  term  gives  the  number  of  hetero- 
zygous and  the  exponent  of  the  second  term  the  number 
of  homozygous  characters.  As  an  example,  suppose  we 
desire  to  know  the  probable  character  of  the  fifth  segre- 
gating generation  (F^)  when  inbred,  if  three  character 
pairs  are  concerned.    Expanded  we  get 

13  +  3  [12(31)] +3  [1  (31)2]  +  (31)3 

Reducing,  we  have  a  probable  fifth-generation  population 
consisting  of  1  heterozygous  for  three  pairs;  93  hetero- 
zygous for  two  pairs;  2883  heterozygous  for  one  pair; 
28,791  homozygous  in  all  three  character  combinations. ' ' 
Of  the  32,768  total  number  of  individuals  in  this  genera- 
tion, 2977,  or  9.09  per  cent.,  are  heterozygous  in  respect  to 
some  characters.  Of  the  98,304  total  number  of  allelo- 
morphic  pairs  involved  in  all  the  individuals  of  this  gen- 
eration, 3072,  or  3.125  per  cent.,  are  heterozygous.  This 
is  the  percentage  which  is  obtained  by  halving  100  per 
cent,  five  times.  It  is  the  per  cent,  of  heterozygous  allelo- 
morphic  pairs  in  all  the  individuals  making  up  the  popu- 
lation as  a  whole  that  follows  curve  1  in  Fig.  24.  The 
per  cent,  of  individuals  heterozygous  in  any  factors  in  any 
generation  inbred  by  self-fertilization  depends  upon  the 
number  of  heterozygous  elements  concerned  at  the  start. 
The  curves  where  1,  5,  10  and  15  heterozygous  allelo- 
morphs are  present  in  the  beginning  are  given  in  Fig.  24. 
These  are  calculated  from  the  formula  given  and  illus- 
trated above.  The  curve  for  the  reduction  in  hetero- 
zygous individuals  where  one  factor  only  is  concerned  at 
the  start,  is  identical  with  the  curve  showing  the  reduction 
in  the  number  of  heterozygous  factors  in  an  inbred  popu- 


92  INBREEDING  AND  OUTBREEDING 

lation  as  a  whole  where  any  nuinber  of  factors  are  con- 
cerned. In  any  case,  almost  complete  homozygosity  is 
reached  in  about  the  tenth  generation.* 

It  must  be  remembered  that  this  reduction  applies  only 
to  the  whole  population,  or  to  a  representative  sample  of 
the  population,  in  which  every  member  is  self  ed,  in  which 
each  individual  is  equally  fertile,  and  in  which  all  the 
progeny  are  grown  in  every  generation.  In  practice  in  an 
inbreeding  experiment,  usually  only  one  individual  in  self- 
fertilization  or  two  individuals  in  brother  and  sister  mat- 
ings  are  used  to  produce  the  next  generation.  Thus  the  rate 
at  which  complete  homozygosity  is  approached  depends  on 
the  constitution  of  the  individuals  chosen.  Theoretically  in 
any  inbred  generation  the  progenitors  of  the  next  genera- 
tion may  either  be  completely  heterozygous  or  completely 
homozygous  or  any  degree  in  between  depending  upon 
chance.  The  only  conditions  which  must  follow  in  self- 
fertilization  is  that  no  individual  can  ever  be  more  hetero- 
zygous than  its  parent,  but  may  be  the  same  or  less.  Thus 
it  is  seen  that  artificial  inbreeding,  as  it  is  practiced,  may 
theoretically  never  cause  any  reduction  in  heterozygosity, 
or  it  may  bring  about  complete  homozygosity  in  the  first 
inbred  generation.  In  other  words,  the  rate  at  which 
homozygosity  is  approached  may  vary  greatly  in  differ- 
ent lines.  However,  as  the  number  of  heterozygous  fac- 
tors at  the  commencement  of  inbreeding  increases  the 
more  nearly  will  the  reduction  to  homozygosity  follow  the 
curve  shown,  because  the  chance  of  choosing  a  completely 

b  Various  formulae  dealing  with  inbreeding  have  been  proposed  and 
discussed  by  Pearson  (177),  Jennings  (102,  104,  105,  106,)  Pearl  (168, 
169,  170,  171,  172),  Fish  (69),  Wentworth  and  Remick  (213),  Robbing 
(186,  187)  which  are  useful  in  predicting  the  character  of  inbred  genera- 
tions when  certain  conditions  are  fulfilled. 


MATHEMATICAL  CONSIDERATIONS         93 

homozygous  or  completely  heterozygous  individual  in  the 
early  generations  will  become  very  much  less. 

TABLE  II 

The  Theoretical  Number  and  Ratio  of  Individualb  in  the  Classes  of 

Different  Degrees  of  Heterozygosity,  After  Recomhination, 

WHEN  Fifteen  Mendelizing  Units  Are  Involved. 


Ratio    of    indi- 

The number'of 

The  total  number  of 

viduals  in  thr 

factors 

in    rp- 

The  total  number  of  hcteroay- 

individuals     in     all 

classes       with 

spcct  to^whirh 

gnus  and   homozygous  factor 

ClaBfl 

the  possible  Mendo- 

different  num- 

the 

difforent 

pairs  ;in  all  the  individuals  in 

No. 

lian  recombinations 
in  Ft  when  15    fac- 
tors are  involved. 

ber  of  hetero- 
zygous   and 
ho  moaygous 

classes 

are: 

each  class: 

factors    -    co- 

Hetero- 

Homo- 

Heterozygous 

HomozvgouB 

efficients  (a-(- 
a),i» 

zygous 

zygous! 

factor  pairs 

factor  pairs 

1 

32,768 

1 

15 

0 

15 

0 

2 

491,520 

15 

14 

1 

210 

16 

3 

3,440,640 

105 

13 

2 

1,365 

210 

4 

14,909,440 

455 

12 

3 

5,460 

1,365 

5 

44,728,320 

1,365 

11 

4 

15,015 

5,460 

6 

98,402,304 

3,003 

10 

5 

30,030 

15,015 

7 

164,003,840 

5,005 

9 

6 

45,045 

30,030 

8 

210,862,080 

6,435 

8 

7 

51,480 

45,045 

9 

210,862,080 

6,435 

7 

8 

45,045 

51,480 

10 

164,003,840 

5,005 

6 

9 

30,030 

45,045 

11 

98,402,304 

3,003 

5 

10 

15,015 

30,030 

12 

44,728,320 

1,365 

4 

11 

5,460 

15,015 

13 

14,909,440 

455 

3 

12 

1,365 

5,460 

14 

3,440,640 

105 

2 

13 

210 

1,365 

15 

491,520 

15 

1 

14 

15 

210 

16 

32,768 

1 

0 

15 

0 

15 

16 

1,073,741,824 

32,768 

15 

15 

245,760 

245,760 

n-fl 

(2n)2 

2" 

n 

n 

mn:2n) 

3^(n.2") 

As  an  example,  in  Table  II  there  is  shown  the  theoreti- 
cal classification  of  the  progeny  of  a  self-fertilized  organ- 
ism which  is  assumed  to  be  heterozygous  ^vith  respect  to 
15  independent  Mendelizing  units.  It  can  be  seen  that  the 
bulk  of  the  individuals  lie  between  classes  6  and  11, 
where  none  of  the  members  is  heterozygous  for  more 
than  10  or  less  than  5  factors.    In  other  words,  any  indi- 


94  INBEEEDING  AND  OUTBREEDING 

vidual  selected  from  this  population  to  be  the  progenitor 
of  the  next  generation  would  most  probably  come  from 
the  middle  classes  and,  therefore,  would  be  heterozygous 
for  only  about  half  as  many  factors  as  its  parent.  The 
chance  that  this  individual  would  not  come  from  the  mid- 
dle classes  between  6  and  11  would  be  about  1  out  of  10. 
The  chance  that  it  would  be  completely  homozygous  or 
completely  heterozygous  is  1  out  of  32,768.  If  20,  instead 
of  15,  factors  were  involved,  the  chance  would  be  1  out  of 
1,048,576.  The  selection  of  such  completely  homozygous 
individuals  would  be  a  remarkable  event.  If,  for  instance, 
a  tobacco  plant,  which  has  24  chromosomes  as  the  haploid 
number,  could  be  obtained  which  was  heterozygous  in  one 
factor  pair  in  each  chromosome  and  this  plant  were  to 
be  self-pollinated  and  the  progeny  grown,  16,777,216 
plants  would  have  to  be  produced  in  order  to  provide  an 
even  chance  of  securing  somewhere  in  the  lot  one  plant 
which  was  homozygous  in  all  the  twenty-four  factors. 
This  number  of  plants  would  require  over  2000  acres  of 
land  as  tobacco  is  grown  in  field  culture. 

This  condition  by  which  the  progenitor  of  each  gen- 
eration in  self-fertilization  tends  to  be  half  as  hetero- 
zygous as  its  parent  holds  true  for  any  number  of  factors 
and  in  every  generation.  Thus  it  can  be  seen  (Table  II) 
that  the  progeny  as  a  whole  have  an  equal  number  of 
heterozygous  and  of  homozygous  factor  pairs  in  respect 
to  those  characters  in  which  the  parent  was  heterozygous. 
So  it  is  that  in  practice  the  reduction  in  heterozygosity 
accompanying  inbreeding  is  greatest  at  first,  rapidly  be- 
comes less  and  finally  ceases  for  all  practical  purposes. 
From  0  at  the  start  the  degree  of  homozygosity,  with  re- 
spect to  a  given  number  of  factors,  increases  to  99  per 
cent,  after  7  generations  of  self-fertilization;  after  12 


MATHEMATICAL  CONSIDERATIONS 


95 


generations  it  is  99.9  per  cent,  and  after  19  generations 
99.999  per  cent. 

Although  nearly  complete  homozygosis  is  theoretically 
brought  about  by  7  generations  of  self-fertilization  the 
attainment  of  absolute  homozygosity  is  a  dillicult  matter 
and  in  practice  it  may  never  be  reached.  The  fact  that 
hereditary  factors  are  distributed  by  the  chromosomes,  so 
that  there  is  not  independent  recombination  among  all 
the  determiners,  enters  as  a  complicating  factor.  Lethal 
factors,  which  prevent  homozygotes  from  appearing,  and 
increased  productivity  of  hybrid  combinations,  also  tend 
to  prevent  the  complete  elimination  of  heterozygosity. 

The  way  in  which  factor  linkage  affects  reduction  to 
homozygosity  may  be  illustrated  by  the  use  of  two  allelo- 
morphic  pairs  of  factors.  Jennings  ^*^^  has  calculated 
the  effect  of  three  generations  of  self-fertilization  upon 
the  population  descending  from  a  dihybrid  when  the  two 
pairs  of  factors  show  a  linkage  relation  of  2  (that  is,  33Vfj 
per  cent.  ^^ crossovers'^)  in  both  sexes;  and  also  when  the 
linkage  is  complete  in  one  sex,  as  in  Drosopliila  where 
there  is  no  * '  crossing  over ' '  in  the  male.  The  proportions 
of  completely  homozygous,  of  completely  heterozygous, 
and  of  mixed  individuals — i.e.,  heterozygous  in  one  pair 
and  homozygous  in  the  other — obtained  after  three  gen- 
erations of  self-fertilization,  are  compared  with  what  is 
expected  when  the  two  factors  are  independent  as  follows : 


Ratio  required 

Two  factors 
independent 

Linkage  ratio 
2  in  botli  sexes 

Linkage 

complete  in 

one  sex,  2  in 

other 

Per  cent,  of  complete  homozygotes . . 

Per  cent,  of  complete  heterozygotes. 

Per  cent,  homozygous  in  one  pair  but 

not  in  other 

7G.56 
1.50 

21.88 
87.50 
12.50 

77.14 
2.14 

20.71 
87.50 
12.50 

78.70 
•A.70 

17.50 

Per  cent,  homozygous  factors 

Per  cent,  heterozygous  factors 

87.50 
12.50 

96  INBREEDING  AND  OUTBREEDING 

It  win  be  seen  from  these  figures  that  the  proportion  of 
complete  homozygotes  and  complete  heterozygotes  is  in- 
creased by  linkage  at  the  expense  of  the  mixed  class.  The 
proportion  either  of  homozygous  or  of  heterozygous  fac- 
tor pairs,  however,  is  unaffected.  It  is  evident  then  that 
just  as  the  reduction  to  homozygosity  by  self-fertilization 
is  independent  of  the  number  of  factors  involved,  in  the 
same  way  it  is  independent  of  the  way  in  which  these 
factors  are  linked  together:  but  in  an  experiment  where 
particular  individuals  are  chosen  as  progenitors  linkage 
of  factors  reduces  the  chance  that  these  will  come  from 
the  median  classes  of  heterozygosity;  hence,  the  rate  at 
which  homozygosity  is  attained  wiU  vary  more  widely 
between  dfferent  lines  if  the  factors  involved  are  par- 
tially linked  than  if  they  are  all  independent.  This  merely 
means  that  some  lines  will  become  uniform  and  lose  the 
stimulus  of  hybridization  in  a  fewer  number  of  genera- 
tions than  will  other  lines  and  that  this  difference  theoreti- 
cally is  increased  by  linkage.  But  the  hastening  of  the 
attainment  of  homozygosity  in  some  lines  is  balanced  by 
delay  in  other  lines,  so  that  on  the  average  the  curve 
of  inbreeding  shown  applies  equally  whether  linkage  of 
factors  is  involved  or  not. 

If  there  were  no  other  controlling  factors  the  reduc- 
tion in  vigor  resulting  from  inbreeding,  in  the  majority 
of  cases,  should  approximate  curve  1  in  Fig.  24  on  the 
assumption  that  hybrid  vigor  or  heterosis  is  associated 
with  heterozygosity.  However,  it  should  not  be  thought 
that  the  amount  of  heterosis  is  perfectly  correlated  with 
the  number  of  heterozygous  factors.  Some  have  more 
of  an  effect  than  others,  and  certain  factors,  when  com- 
bined together,  may  have  a  cumulative  effect.    Moreover, 


MATHEMATICAL.  CONSIDERATIONS         97 

since  the  heterozygous  individuals  are  more  vigorous  than 
the  homozygous,  selection  either  unconscious  or  purpose- 
ful would  favor  the  more  heterozygous  so  that  actual 
approach  to  homozygosity  is  quite  likely  not  to  proceed 
at  as  fast  a  rate  as  the  theoretical  curve  would  indicate. 

Self- fertilization  is  the  quickest  and  surest  means  of 
obtaining  complete  homozygosity  for  the  reason  that 
whenever  any  pair  of  allelomorphs  becomes  homozygous 
it  must  always  remain  so,  as  long  as  self-fertilization  takes 
place,  whereas  in  brother  and  sister  mating  a  homozygote 
may  be  mated  to  a  heterozygote.  The  approach  to  homo- 
zygosity in  self-fertilization  when  one  pair  of  contrasted 
characters  is  considered  and  fecundity  does  not  vary  pro- 
ceeds as  follows : 

Generation  F^  F^  F^       F^ 

Parent  type  A  A  0  1/4  3/8  7/16 1/2  AA^  =.492 

Parent  type  aa  0  1/4  3/8  7/16 1/2  aa,  =.492 

Hybrid  type  Aa  1  1/2  1/4  1/8 0  Aa^  =.008 

In  brother  and  sister  mating  the  procedure  is 
as  follows; 

Generation  F^    F^  F^  F^  F^ 

Parent  type  ^^  0     1/4  2/8  5/16     11/32 1/2  ^^,,=.490+ 

Parent  type  aa  0    1/4  2/8  5/16    11/32 1/2  aa^,  =.490+ 

Hybridtype^a  1     1/2  2/4  3/8        5/16 0  Aa^^=mS-\- 

These  figures  have  definite  numerical  relations  to  each 
other  and  formulae  have  been  obtained  by  Jennings  ^^^  for 
calculating  the  condition  in  any  generation.  It  will  be  seen 
from  the  above  figures  that  6  generations  of  self-fertiliza- 
tion are  more  effective  than  17  generations  of  brother  and 
sister  matings  in  bringing  about  homozygosis. 

Of  parent  by  offspring  matings  there  are  several 
7 


98  INBEEEDING  AND  OUTBEEEDING 

meth-ods  which  may  be  carried  on  with  different  results. 
If  the  individuals  are  mated  at  random  to  all  the  differ- 
ent types  of  parents,  homozygous  and  heterozygous,  the 
effect  is  the  same  as  in  brother  and  sister  mating.  Cousin 
matings,  which  may  proceed  either  as  single  or  double 
first-cousin  matings  or  even  more  distant  unions,  may  be 
equally  or  less  effective;  but  all  tend  towards  the  same 
end;  heterozygosity  is  ultimately  eliminated  and  homo- 
zygosity prevails.  In  this  way  there  is  a  difference  be- 
tween selective  mating  and  random  mating.  Continued 
selective  mating  is  necessary  to  bring  about  homozygos- 
ity. Intermittent  inbreeding  alternating  with  periods 
of  outcrossing  which  is  the  prevailing  state  of  affairs 
with  many  organisms  cannot  maintaiu  any  high  degree 
of  homozygosity. 

In  self-fertilization  the  reduction  in  heterozygous 
allelomorphs  in  a  population  as  a  whole  follows  curve  1  in 
Fig.  24,  irrespective  of  the  number  of  factors  concerned, 
as  stated  before,  provided  that  a  random  sample  of  all 
the  different  classes  of  individuals  are  selfed  and  become 
progenitors  of  the  next  generation  and  that  there  is 
equal  productiveness  and  equal  viability.  If  the  hetero- 
zygotes  are  more  productive,  as  in  many  cases  they  are, 
the  reduction  to  complete  homozygosity  will  be  delayed. 

Artificial  self-fertilization  in  naturally  crossed  species, 
then,  brings  about  the  same  condition  as  prevails  in 
naturally  selfed  species.  The  great  variability  of  a  cross- 
fertilized  species  gives  way  to  the  more  uniform  and 
stable  condition  characteristic  of  naturally  self-fertilized 
organisms.  The  uniformity  brought  about  by  inbreediQg 
is  thus  due  to  a  reduction  of  the  genetical  variability. 
Inbreeding  affects  physiological  or  developmental  vari- 


MATHEMATICAL  CONSIDERATIONS         99 

ability  only  indirectly  by  changing  the  vigor  of  the  organ- 
isms so  that  they  may  react  differently  to  dilTerent 
environments. 

Assuming,  then,  that  the  loss  of  the  stimulation  ac- 
companying heterozygosity  is  correlated  with  the  reduc- 
tion in  the  number  of  heterozygous  factors  we  should 
expect  to  find  the  decrease  of  heterosis  greatest  in  the  first 
generations,  rapidly  becoming  less  until  no  further  loss 
is  noticeable  in  any  number  of  subsequent  generations 
of  self-fertilization,  and  that  on  the  average  the  decrease 
will  become  negligible  from  the  seventh  to  the  twelfth 
generation  and  from  then  on  no  further  marked  change 
will  take  place.  Segregation  of  characters  and  appearance 
of  new  types  and  reduction  in  variability  will  also  follow 
the  same  course.  Some  cases  are  to  be  expected  in  whioh 
stability  is  reached  earlier,  and  some  cases  in  which  it  is 
reached  later;  or,  theoretically  it  may  never  be  reached. 
With  these  points  in  mind,  let  us  see  what  are  the  actual 
results  of  long-continued  inbreeding. 


CHAPTER  VI 

INBREEDING    EXPERIMENTS    WITH    ANIMALS 

AND  PLANTS 

Doubtless  discussion  has  been  rife  since  the  dawn  of 
civilization  as  to  the  actual  effect  of  more  or  less  close 
intermating  in  the  various  breeds  of  domestic  animals, 
since  stock-raising  was  one  of  the  earliest  arts  and  was 
brought  to  a  high  degree  of  perfection  by  the  ancient 
Semitic  nations.  One  may  surmise,  from  the  rules  they 
made  against  the  marriage  of  near  relatives,  that  the  pro- 
ponents of  cross  breeding  had  the  best  of  the  argu- 
ment ;  but  it  is  hardly  likely  that  their  practice  was  any- 
thing more  than  rule-of -thumb  adopted  after  a  variety  of 
casual  observations.  At  any  rate,  controversy  is  still  spir- 
ited, and  one  reads  the  opinion  of  stock-breeding  authori- 
ties without  arriving  at  any  definite  knowledge  of  the 
problems.  Their  results  are  confusing,  and  the  only  con- 
clusion one  may  reach  from  their  perusal  is  the  wholly 
unsatisfactory  one  that  close  mating,  as  a  system  of  breed- 
ing, has  both  advantages  and  disadvantages.  Without 
question,  it  has  had  great  value  in  fixing  certain  desirable 
types.  Some  breeds,  as  a  whole,  and  many  individual 
herds,  owe  their  uniformity  in  conformation  and  perform- 
ance in  a  large  measure  to  close  inbreeding  accompanied 
by  rigid  selection.  At  the  same  time,  it  must  be  recog- 
nized that  certain  evil  effects  may  result  from  close  inter- 
mating. These  effects  have  been  frequently  expressed  in 
lessened  constitutional  vigor,  greater  susceptibility  to 
disease,  reduced  fecundity,  and,  in  some  cases,  even  in 

100 


/ 


INBREEDING  EXPERIMENTS  101 

decreased  size  and  in  the  appearance  of  definite  abnormal 
or  pathological  conditions. 

Obviously  it  is  not  possible  to  formulate  any  definite 
rule  by  which  to  judge  under  what  condition  the  good 
effects  of  inbreeding  may  be  expected  to  outweigh  the 
evil,  by  generalizing  from  a  series  of  isolated  facts.  What 
is  needed  is  controlled  experimentation  to  determine  just 
what  inbreeding  involves,  and  interpretation  of  the  re- 
sults in  keeping  with  general  biological  knowledge.  Dar- 
wiii38,  39    ^jjg    ^i^g    gj.g|-    ^Q    appreciate    this.      After 

endeavoring  rather  unsuccessfully  to  generalize  on  the 
subject  with  a  collection  of  published  records  as  a  basis, 
he  himself  carried  on  a  series  of  inbreeding  experiments 
on  plants  extending  over  a  period  of  eleven  years.  Plants 
were  probably  selected  as  the  material  with  which  to  work 
primarily  because  the  experiments  could  be  carried  out 
easily  and  with  little  expense.  But  there  is  another  rea- 
son why  plants  serve  the  purpose  better  than  animals  in 
such  an  investigation;  the  animals  commonly  available 
are  bisexual;  hence,  they  cannot  be  inbred  as  intensively 
as  hermaphroditic  plants.  Nevertheless,  Darwin's  ex- 
periments served  to  stimulate  more  interest  in  the  subject 
among  zoologists  than  among  botanists,  and  the  quanti- 
tative experiments  carried  on  during  the  next  thirty 
years  w^ere  nearly  all  upon  animals. 

The  rat  has  been  a  favorite  species,  it  having  served 
as  material  for  the  extended  researches  of  Crampe,'^* 
Ritzema-Bos  '^^  and  King.^^**>'  ^^o.  121  rpj^g  grgt  two  investi- 
gations, together  with  that  of  Weismann  and  von 
Guaita,^^'  ^'^  on  mice  have  been  the  classic  examples  of  the 
adverse  effects  of  inbreeding. 

Crampe^s  experiments  started  with  a  litter  of  five 


102         INBREEDING  AND  OUTBREEDING 

young,  obtained  by  crossing  an  albino  female  with  a  wnite 
and  gray  male.  These  animals  were  inbred  in  various 
degrees  for  seventeen  generations.  During  the  experi- 
ment many  rats  showed  great  susceptibility  to  disease, 
divers  kinds  of  abnormalities,  diminished  fertility,  and 
increased  total  sterility. 

Similarly  Ritzema-Bos  started  his  investigations  with 
a  litter  of  twelve  rats  obtained  by  crossing,  this  time  an 
albino  female  with  a  wild  Norway  male.  This  stock  was 
inbred  in  different  ways  for  six  years,  during  which  time 
he  claimed  to  have  obtained  about  thirty  generations.  His 
results  did  not  corroborate  those  of  Crampe  in  so  far  as 
susceptibility  to  disease  or  appearance  of  malformations 
are  concerned,  but  there  was  a  gradual  decrease  in  size 
of  litter  and  a  gradual  increase  in  percentage  of  infertile 
matings,  as  is  shown  in  the  following  table : 

Year  of  inbreeding 1  2  3      4      5      6 

Ave.  number  ia  litter 7.5  7.1  7.1    6.5   4.2    3.2 

Per  cent,  infertile 

matings 0.0  2.6  5.617.4  50.0  41.2 

These  investigations,  in  spite  of  the  habit  biologists 
have  of  citing  them,  are  not  calculated  to  settle  the  ques- 
tion they  undertook  to  answer.  Ritzema-Bos  himself 
criticizes  those  of  Crampe,  because  he  believes  them  to 
have  been  started  with  a  weak  strain.  Miss  King,  how- 
ever, thinks  the  weakness  of  these  rats,  as  indicated  by 
their  susceptibility  to  disease,  the  appearance  of  mal- 
formations, and  their  tendency  to  sterility,  was  due  to 
the  conditions  under  which  they  were  kept.  She  had  a 
similar  experience  during  the  earlier  part  of  her  own 
experiments,  and  found  that  inadequate  nourishment  was 
largely  the  cause.    But  it  is  not  for  this  reason  that  we 


INBREEDING  EXPERIMENTS  103 

feel  that  both  of  these  experiments  should  be  disregarded. 
Each  ivas  started  with  hybrid  stock,  and  such  experi- 
ments with  hybrid  stock  bring  in  an  additioyial  compli- 
cation, Mendelian  recombination.  The  only  type  of 
investigation  on  bisexual  animals  calculated  to  offer  criti- 
cal evidence  on  the  effect  of  inbreeding  per  se  must  be 
carried  on  with  stock  which  has  already  been  inbred  long 
enough  to  reduce  the  genetic  constitution  of  the  animals 
to  an  approximately  homozygous  condition.  Then,  and 
then  only,  can  the  effect  of  more  extended  inbreeding  be 
determined  without  confusion  as  to  the  interpretation 
of  the  results. 

Miss  King  has  pointed  out  a  part  of  the  difTiculties 
involved  in  starting  with  a  hybrid  stock.  In  one  of  her 
experiments  the  progeny  of  a  cross  between  a  wild  Nor- 
way rat  and  an  albino  was  inbred  for  several  generations. 
She  found  that  while  the  majority  of  the  F-^  females  were 
fertile,  at  least  25  per  cent,  of  the  Fo  females  were  com- 
pletely sterile  and  10  per  cent,  of  those  which  did  breed 
cast  only  one  or  two  litters.  In  the  strains  extracted  from 
this  cross  there  was  variation  in  degree  of  fertility,  but 
none  was  found  which  exhibited  the  high  degree  of  fer- 
tility usually  existent  in  the  albino  rat.  No  endeavor  to 
select  fertile  strains  was  made  and  one  cannot  say  whether 
or  not  rigid  selection  would  have  isolated  them,  but  the 
researches  of  Detlefsen^'  on  hybrids  of  the  genus  Cavia 
to  which  the  common  guinea-pig  belongs  indicate  this  to 
be  a  probability.  These  investigations  as  well  as  those  of 
Easf^^  on  the  genus  Nicotiana  show  conclusively  that 
various  hereditary  factors  are  involved  in  the  partial 
sterility  exhibited  in  many  species  crosses,  and  that  tliese 
factors  may  be  expected  to  recombine  in  the  usual  manner. 


104         INBREEDING  AND  OUTBREEDING 

It  is  more  than  a  mere  assumption,  then,  if  a  great  part  of 
the  sterility  found  by  Crampe  and  Ritzema-Bos  is  attrib- 
uted to  the  same  cause. 

The  investigations  of  Weismann  and  von  Guaita  are 
hardly  more  satisfactory.  Weismann  inbred  a  stock  of 
white  mice  for  twenty-nine  generations,  and  found  the 
average  number  of  young  for  the  three  10-gen.  periods 
to  be  6.1,  5.6  and  4.2.  Where  this  stock  originated,  and 
what  method  of  inbreeding  was  followed,  we  are  not  told. 
Presumably  the  gross  result  was  a  slight  decrease  in  fer- 
tility, coincident  with  the  amount  of  inbreeding,  but  even 
this  is  not  certain.  As  King  points  out,  the  average  num- 
ber of  litters  under  observation  in  the  first  two  genera- 
tions was  twenty-two ;  in  the  last  nine  generations,  three. 
Clearly  there  was  greater  opportunity  of  selecting 
healthy  breeding  stock,  as  well  as  a  lower  probable  error, 
in  the  earlier  part  of  the  experiment,  and  this  might 
account  for  the  slight  difference  in  fertility  found.  Von 
Guaita  crossed  some  of  these  highly  inbred  mice  with 
Japanese  waltzers  and  then  inbred  for  six  generations. 
He  reports  the  average  number  of  young  in  the  successive 
generations  as  4.4,  3.0,  3.8,  4.3,  3.2  and  2.3 ;  but  in  view  of 
the  vigor  almost  always  expressed  in  the  F^  generation  of 
such  crosses  one  is  inclined  to  doubt  the  pertinence  of 
these  figures  to  the  inbreeding  problem. 

Although,  as  has  been  pointed  out,  there  is  good  rea- 
son for  disallowing  the  claim  of  these  much  cited  experi- 
ments on  mammals  as  proofs  of  the  adverse  effects  of 
inbreeding  through  consanguinity  alone,  there  is  no  inten- 
tion of  denying  the  isolation  of  individuals  characterized 
by  undesirable  qualities  from  mixed  strains  by  means  of 
Mendelian  recombination.    Perhaps  it  is  not  wise  even  to 


INBREEDING  EXPERIMENTS  105 

maintain  the  impossibility  of  injury  to  any  strain  of  any 
species  through  inbreeding  per  se,  but  it  is  proper  to  say 
that  the  evidence  in  favor  of  it  is  practically  nil. 

Doubtless  we  could  make  our  case  more  convincing  to 
the  stockman  could  the  enormous  number  of  really  well- 
kept  herd  records  be  cited  and  analyzed.  But  it  is  not 
possible  at  present  to  say  whether  many  of  these  records 
satisfy  the  requirements  of  modern  genetic  research. 
This  is  a  task  which  must  be  left  to  the  breeding  organiza- 
tions of  the  future.  We  can  appeal  at  present  to  only  two 
investigations  on  mammals  where  the  effect  of  Mendelian 
recombination  has  been  largely  eliminated;  and  these 
again  are  on  small  mammals,  the  rat  and  the  guinea-pig. 

The  first  of  these  investigations  to  be  reported  was 
that  of  King.^^^'  ^^o,  121  j-|-  ^r^g  started  with  a  litter 
of  four  slightly  undersized  but  otherwise  normal  albino 
Norway  rats,  two  males  and  two  females.  From  these 
females  two  lines,  A  and  J5,  were  carried  on  for  twenty- 
five  generations  by  mating  brother  and  sister.  In 
the  earlier  generations  practically  all  of  the  females  were 
used  for  breeding,  but  in  every  generation  after  the  sixth 
about  twenty  females  were  selected  from  approximately  a 
thousand  available  young. 

At  first  the  inbred  rats  exhibited  many  of  the  defects 
reported  by  Crampe.  Numerous  females  were  either  ster- 
ile or  produced  but  one  or  two  small  litters.  Other  ani- 
mals were  characterized  by  low  vitality,  dwarfing,  and 
malformations.  Stock  rats  exhibiting  the  same  charac- 
teristics at  this  time,  however,  led  to  a  change  in  the  food, 
following  which  the  **dire  effects  of  inbreeding'^  prac- 
tically disappeared.  Whether  this  improvement  in  the 
colony  was  due  entirely  to  the  change  of  diet  or  may  be 


106         INBREEDING  AND  OUTBREEDING 

attributed  partly  to  selective  eUmination  of  the  weaker 
rats  cannot  be  determined.  We  are  inclined  to  agree  with 
Miss  King  in  giving  greater  weight  to  the  first  factor, 
though  for  a  reason  which  she  does  not  mention.  The 
general  success  of  Miss  King^s  whole  investigation  we 
believe  to  be  due  largely  to  the  fact  that  the  experiments 
were  started  with  stock  rats  which  already  must  have 
been  very  closely  inbred  and  therefore  in  an  approxi- 
mately homozygous  condition. 

From  the  seventh  generation  on,  selection  was  made 
on  the  new-bom  young  with  general  vigor  as  the  basis, 
but  the  two  lines  were  selected  differently.  In  line  A  only 
litters  having  an  excess  of  males  were  selected  to  serve 
as  the  progenitors  of  the  succeeding  generation,  while  in 
line  B  the  reverse  was  the  case.  The  general  result  was 
to  show  that  the  normal  sex  ratio  in  this  species,  105  males 
to  100  females,  can  be  changed.  At  the  end  of  nineteen 
generations  of  selection,  line  A  had  produced  litters  hav- 
ing a  sex  ratio  of  122.3  males  to  100  females,  and  line  B 
had  produced  litters  having  a  sex  ratio  of  81.8  males  to 
100  females.  From  these  facts  there  is  no  doubt  but  that 
lines  having  an  hereditary  tendency  to  produce  different 
sex  ratios  can  be  isolated,  but  there  is  no  evidence  what- 
ever in  favor  of  the  theory  of  Diising  proposed  in  1883  to 
the  effect  that  inbreeding  by  lessening  the  vitality  of  the 
mother  increases  the  percentage  of  male  young.  The 
change  in  the  sex  ratio  was  made  in  two  generations. 
After  that  the  effect  of  selection  ceased.  Such  a  result 
not  only  militates  against  attributing  the  changed  ratios 
to  inbreeding  itself,  but  indicates  that  a  relatively  small 
number  of  Mendelian  factors  are  involved  in  the  control. 

The  effect  of  continuous  inbreeding  on  body  weight  is 


INBREEDING  EXPERIMENTS 


107 


shown  in  Fig.  25.  This  graph  is  constructed  from  data 
collected  from  the  records  of  males  of  line  A,  but  graphs 
constructed  from  the  records  of  the  females  of  this  line 
and  from  males  and  females  of  line  B  do  not  differ  from 


Growth  in  bo<^y  wci 
H   S«ries  A 


ighl    Albtno  Rat 
Males 


PTn 


120    140    160    180  200  220  240  260  260 


Age  in  days 


Fig.  25. — Graphs  showing  the  increase  in  the  body  weight  with^age  for  males  of  inbred 
albino  rats.  (Series  A.)  A,  graph  for  the  males  of  the  seventh  to  the  ninth  generati<  r.u 
inclusive;  B,  graph  for  the  males  of  the  tenth  to  the  twelfth  generatinns  inclusive;  C,  graph 
for  the  males  of  the  thirteenth  to  the  fifteenth  generations  iucluhive;  D,  giaph  for  the  n.ult* 
of  the  Grst  six  inbred  generations.     (After  King.) 

it  in  any  essential  feature.  Curve  D  is  further  evidence 
for  concluding  that  the  animals  of  the  first  six  generations 
suffered  from  malnutrition,  since,  as  Miss  King  notes,  it 
is  preposterous  to  suppose  that  these  animals  could  have 
given  rise  to  the  very  large  individuals  represented  by 


108         INBREEDING  AND  OUTBREEDING 

curve  A  \1  it  really  represented  their  true  body  weight. 
How  favorably  these  inbred  strains  compare  with  stock 
animals  is  shown  in  Fig.  26. 

Paralleling  the  results  obtained  for  body  weight  were 


20    40    eo 


320  340  360  380  400  420  440  460  460 


Fig.  26. — Graphs  showing  the  increase  in  the  weight  of  the  body  with  age  for 
different  series  of  male  albino  rats.  A,  graph  constructed  from  Donaldson's  data  for  stock 
albinos;  B,  graph  for  males  belonging  in  the  seventh  to  the  fifteenth  generations  of  the 
two  series  of  inbreds  combined;  C,  graph  constructed  from  data  for  a  selected  series  of 
stock  albinos  used  as  controls  for  the  inbred  strain;  D,  graph  for  males  belonging  in  the 
first  six  generations  of  the  two  series  combined.     (AfterKing.) 

those  upon  fertility  and  constitutional  vigor  as  judged  by 
longevity.  Neither  was  reduced  by  inbreeding;  in  fact, 
there  seems  to  be  no  doubt  but  that  there  was  a  significant 
increase  in  both  cases.  There  was  a  slight  but  definite 
increase  in  fertility  as  is  evident  if  one  plots  the  theoreti- 
cal curve  which  fits  the  experimental  curve  for  litter  size 


INBREEDING  EXPERIMENTS 


109 


throughout  the  course  of  the  experiment  (Fig.  27).  The 
whole  series  of  inbreds  compares  well  in  this  respect  with 
stock  albinos  for  which  the  average  litter  size  is  G.7. 
Further  there  was  a  notable  increase  in  longevity  in  line 
A  and  a  marked  increase  in  line  B. 


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Fia.  27. — Graph  showing  the  average  Bise  of  litters  produced  in  Bucceseiye  generatione  of 
inbreeding  albino  rats  by  brother  and  sieter  matings.     (After  King.) 

The  interpretation  of  these  experiments  is  wholly  in 
accordance  with  Mendelian  theory.  Starting  with  stock 
rats  which  from  previous  close  breeding  had  already  been 
reduced  to  a  high  degree  of  homozygosity,  inbreeding  had 
the  tendency  to  accentuate  this  purity  of  type  and  to 
segregate  slight  differences.  By  selection  vigorous  uni- 
form strains  were  built  up,  strains  somewhat  larger,  more 


110         INBEEEDING  AND  OUTBREEDING 

fertile  and  longer  lived  than  many  strains  of  stock  rats. 
It  is  clear  that  this  was  the  result  of  Mendelian  recom- 
bination for  the  two  lines  A  and  B  were  in  the  end  some- 
what different.  The  rats  of  line  A  were  slightly  more 
fertile,  attained  sexnal  maturity  earlier,  and  lived  longer 
than  those  of  line  B,  If  this  evidence  were  not  sufficient,  it 
is  supplemented  by  the  fact  that  variability  gradually  be- 
came reduced  during  the  progress  of  the  inbreeding. 

The  investigations  of  the  effects  of  inbreeding  on  the 
guinea-pig,  to  which  we  have  referred,  were  begun  in 
1906  by  G.  M.  Rommel  of  the  United  States  Department 
of  Agriculture.  In  recent  years  the  work  has  been  in 
charge  of  SewaU  Wright,  who  has  made  a  very  illumi- 
natiag  analysis  of  the  results  obtained. 

This  series  of  experiments  was  started  with  thirty- 
three  pairs  of  stock  animals  which  had  been  more  or  less 
inbred  previously.  Although  maintained  exclusively  by 
mating  sister  with  brother,  sixteen  of  these  families  were 
in  existence  at  the  close  of  1917  after  some  twenty  gen- 
erations of  the  closest  inbreeding. 

Considered  as  a  whole  this  inbred  race  shows  distinct 
evidence  of  having  declined  in  every  character  connected 
with  vigor.  The  litters  are  smaller  and  are  produced 
more  irregularly.  The  per  cent,  of  mortality  both  in  utero 
and  between  birth  and  weaning  has  increased.  The  birth 
weights  are  lower  and  the  rate  of  growth  slower  than  in 
control  stock.  In  spite  of  these  facts,  however,  one  is 
forced  to  the  conclusion  that  these  results  are  not  the 
effect  of  inbreeding  as  a  direct  cause,  but  are  to  be  attrib- 
uted to  Mendelian  segregation. 

There  are  pronounced  differences  between  the  various 
families.    Some  are  still  very  vigorous,  comparing  favor- 


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INBREEDING  EXPERIMENTS  111 

ably  with  the  original  stock;  others  degenerated  so  rap- 
idly that  they  soon  became  extinct  in  spite  of  every  etl'ort 
to  prevent  such  a  catastrophe.  Among  the  families  still 
in  existence,  there  is  even  evidence  that  vigor  as  a  general 
term  may  be  divided  into  various  causative  factors  and 
that  these  factors  may  be  combined  in  various  ways,  liy 
grading  each  family  for  various  characters  connected 
with  vigor  of  growth  and  reproduction  and  then  classi- 
fying each  family  in  numerical  order  for  each  separate 
character,  Wright  has  been  able  to  show  conclusively 
that  there  ar^  many  hereditary  factors  which  alfect  fer- 
tility, growth  and  vitality  and  that  almost  any  combina- 
tion of  these  characters  may  become  fixed  in  a  family 
through  inbreeding. 

A  little  later  we  shall  have  occasion  to  speak  of  sev- 
eral noteworthy  end  results  obtained  by  inbreeding  the 
larger  domestic  mammals,  but  no  further  discussion 
seems  advisable  in  this  place  because  of  the  lack  of  quan- 
titative data.    A  similar  statement  holds  for  birds. 

The  fruit  fly,  Drosopliila  ynelanogaster^  is  the  only 
insect  which  has  been  used  for  extended  experiments  on 
effects  of  inbreeding,  although  there  are  numerous 
examples  on  record  where  an  importation  of  a  rela- 
tively small  number  of  individuals  has  resulted  in  an 
overwhelming  increase — witness  the  gypsy  moth  in 
New  England. 

Castle  and  his  co-workers  ^^  bred  Drosophila  for 
many  generations  by  continuous  brother  and  sister  mat- 
ings.  After  fifty-nine  generations  of  this  close  inbreed- 
ing the  fertility  did  not  appear  to  be  reduced  below  that 
shown  by  the  original  stock,  although  it  was  increased 
by  crosses  between  certain  inbred  lines.    There  was  some 


112         INBREEDING  AND  OUTBREEDING 

indication  of  reduction  in  size  when  inbred  flies  were 
compared  with  random  mated  stock  reared  under  the 
same  conditions.  Far  from  being  exterminated  by  in- 
breeding, however,  the  flies  at  the  end  of  the  experi- 
ment were  apparently  fully  equal  to  those  with  which  it 
was  begun,. 

These  experiments  showed  clearly  that  inbreeding 
results  in  strains  of  unequal  fertility.  The  less  fertile 
tended  to  be  eliminated  by  differential  productiveness,  so 
that  only  the  more  fertile  remained.  The  occurrence  of 
absolute  sterility  was  pronounced  in  the  first  part  of  the 
experiment,  but  almost  entirely  disappeared  in  the  later 
generations.  The  figures  as  calculated  from  their  table 
are  as  follows : 

Per  cent. 

of  matings 

totally  sterile 

Generations      6  to  24 17.80 

Generations    25  to  42 18.47 

Generations    43  to  59 3.37 

Such  a  result  is  to  be  expected  when  it  is  remembered 
that  inbreeding  produces  homozygous  individuals,  and 
these,  whenever  sterile,  are,  of  course,  eliminated. 

Moenkhaus,^^^  Hyde,^^  and  likewise  Wentworth,^^^ 
by  similar  inbreeding  experiments  with  Drosophila 
found  sterility,  though  increased  in  the  first  stages  of 
inbreeding,  tended  to  be  eliminated  after  the  process 
was  long  continued. 

The  only  other  experiments  on  invertebrates  which 
ought  to  be  cited  here  are  those  of  Whitney  ^^^  and  A.  F. 
ShuU  ^^^  on  the  rotifer  Hydatina  senta.  Both  of  these 
investigators  found  that  inbreeding  had  a  considerable 
adverse  effect  on  the  size  of  family,  number  of  eggs  laid 


INBREEDING  EXPERIMENTS  113 

per  day,  rate  of  growth,  and  variability.  The  proi>er  iii- 
tei-pretation  of  their  resultis  is  somewhat  obscure,  unless 
one  postulates  the  origin  of  frequent  mutations.  Thu 
number  of  generations  bred  and  the  number  of  families 
under  observation  were  not  sufficient  to  demonstrate  the 
segregation  of  differences  in  these  characteristics,  thuugh 
this  is  to  be  expected  since  these  qualities  are  sympto- 
matic of  general,  vigor  and  general  vigor  was  increased 
by  crossing.  The  difficulty,  however,  lies  in  the  fact  that 
continued  parthenogenetic  multiplication  which  is  possible 
in  Hydatina  had  the  same  effect  as  continued  inbreeding. 
Shull  introduces  the  interesting  speculation  that  this  sim- 
ilarity is  due  to  a  gradual  adjustment  of  nucleus  to  cyto- 
plasm during  the  asexual  propagation — this  being  as- 
sumed to  bring  about  the  same  results  as  a  gradual 
approach  toward  homozygosis.  We  are  inclined  to  at- 
tribute both  changes  to  environmental  causes,  believing 
that  if  a  proper  change  in  diet  had  been  made  vigor  would 
have  been  maintained. 

While  we  are  not  justified  in  concluding  from  these 
experiments  that  inbreeding  accompanied  by  rigid  selec- 
tion will  be  beneficial  to  bisexual  animals,  they  certainly 
show  close  mating  is  not  invariably  injurious.  They  in- 
dicate that  the  results  of  inbreeding  depend  more  upon 
the  genetic  composition  of  the  individuals  subjected  to 
inbreeding  rather  than  upon  any  pernicious  intluenc^e  in- 
herent in  the  process  itself;  and,  as  will  be  emphasized 
more  strongly  later,  it  is  a  wholly  different  matter 
whether  inbreeding  results  injuriously  through  the  inlieri- 
tance  received,  or  whether  consanguinity  itself  is  respon- 
sible. Yet  such  a  status  for  the  problem  is  unsatisfactory. 
The  experiments  on  animals  bring  to  light  no  facts  which 

8 


114         INBEEEDING  AND  OUTBREEDINa 

may  not  be  interpreted  as  the  result  of  Mendelian  factor 
recombination ;  but  if  one  were  to  base  his  judgment  on 
them  alone,  he  could  not  truthfully  make  the  didactic  state- 
ment that  inbreeding  per  se  is  not  injurious.  There  would 
ever  be  the  uncertainty  with  which  the  additional  variable 
bisexuality  always  encumbers  a  genetic  experiment.  For- 
tunately, we  may  turn  to  the  numerous  experiments  on 
hermaphroditic  plants  for  the  deciding  vote. 

Many  wild  species  and  cultivated  varieties  of  plants 
are  almost  invariably  self -fertilized,  and  apparently  lack 
nothing  in  vigor,  productiveness  or  ability  to  survive. 
Amon^  wild  plants  many  species  of  the  family  Legumi- 
nosae,  among  cultivated  plants — ^wheat,  rice,  barley,  oats, 
tobacco,  beans,  tomatoes — are  types  characterized  by  very 
nearly  continuous  self-fertilization,  and  these  plants  are 
in  no  immediate  danger  of  extinction. 

On  the  other  hand,  the  majority  of  the  higher  plants 
is  provided  with  devices  which  promote  natural  cross- 
pollination,  and  show  definite  injurious  effects  when  in- 
bred artificially.  Even  species  which  are  characteristi- 
cally self-fertilized  are  crossed  occasionally.  This,  to- 
gether with  the  fact  that  nearly  all  plants  and  animals 
are  benefited  by  crossing,  led  Knight  as  early  as  the  close 
of  the  eighteenth  century  to  believe  self-fertilization  is 
not  a  natural  process  and  always  produces  more  or  less 
injurious  results.  His  views  were  summed  up  in  the  state- 
ment, *  *  nature  intended  that  a  sexual  intercourse  should 
take  place  between  neighboring  plants  of  the  same  spe- 
cies.'* Darwin,  fifty  years  later,  basing  his  conclusions 
upon  observation  of  animals  and  direct  experimentation 
with  plants,  was  even  more  radical,  and  concluded  that 
** nature  abhors  perpetual  self-fertilization." 


INBREEDING  EXPERIMENTS  115 

Darwin  compared  self-fertilizefl  plants  witli  iiit<'r- 
crossed  plants  in  many  different  species.  In  tlie  majority 
of  cases  the  self -fertilized  plants  were  clearly  inferior 
to  the  crossed  plants.  These  facts  led  to  the  belief  that 
the  evil  effects  of  inbreedin.2:  kept  on  accnmnlatinsr  nntil 
eventually  a  plant  or  animal  continuously  reproducing,^  in 
that  manner  was  doomed  to  extinction.  His  own  ex]>eri- 
mental  results  came  far  short  of  proving  such  an  assump- 
tion, however.  The  two  plants  with  which  inhrc'Oflinir  was 
practiced  the  longest — Ipomea  and  Mlmnlu^ — showed 
very  little  further  loss  of  vigor  after  the  first  generation. 
What  the  experiments  did  show,  most  clearly,  was  segre- 
gation of  the  inbred  stock  into  types  differing  in  tlieir 
ability  to  grow  as  well  as  in  minor,  visible,  hereditary 
characters.  In  both  species  plants  appeared  wliich  were 
superior  to  other  plants  derived  from  the  same  source, 
some  being  equal  or  even  superior  in  vigor  to  the  original 
cross -pollinated  stock.  The  inbred  plants  differed  from 
the  original  material  most  noticeably  in  the  uniformity 
of  visible  characters.  Darwin  ^s  gardener  stated  it  was  not 
necessary  to  label  the  plants,  as  the  different  lines  were 
so  distinct  from  each  other  and  so  uniform  among  them- 
selves they  could  easily  be  recognized. 

After  several  generations  of  inbreeding,  Dar^vin 
found  it  made  no  difference  in  the  resulting  vigor  whether 
the  plants  in  an  inbred  lot  were  selfed  or  were  crossed 
among  themselves.  This  he  correctly  ascribed  to  the  fact 
that  the  members  of  such  an  inbred  strain  had  become 
germinally  alike.  With  less  justice  he  attributed  this 
approach  to  similarity  in  inherited  qualities  to  tlie  fact 
that  the  plants  were  grown  for  several  generations  under 


116         INBEEEDING  AND  OUTBREEDING 

the  same  conditions,  but  it  is  easy  to  see  why  he  held  so 
tenaciously  to  this  view  if  one  remembers  the  faith  he  had 
in  the  effect  of  environment  on  organisms.  Such  a  view 
he  deemed  supported  by  the  fact  that  crosses  of  selfed 
lines  with  the  intercrossed  lines  (also  inbred,  but  to  a  less 
degree)  did  not  give  as  great  increases  in  growth  as 
crosses  of  selfed  lines  with  fresh  stock  from  other  local- 
ities. His  crosses  between  inbred  lines  did  give  noticeable 
increases  in  growth,  however,  in  many  cases  equaling  the 
original  variety.  This  is  well  illustrated  by  Dianthus,  in 
which  the  selfed  line  was  crossed  both  with  the  inter- 
crossed line  and  with  a  fresh  stock.  The  ratio  of  each 
crossed  population  to  the  selfed  population  in  height, 
number  of  seed  capsules,  and  weight  of  seed  produced  is 
as  follows : 

Selfed  Selfed 

X  X 

Intercrossed  Fresh  Stock 

Height,  comipared  to  selfed  plants 100 :95  100 :81 

No.  capsules  oompared  to  selfed  plants 100:67  100:39 

Weight  of  seed  compared  to  selfed  plants 100 :73  100 :33 

With  Darwin  we  still  attribute  the  greater  increase 
of  vigor  in  crosses  of  distinct  stocks  to  a  greater  germinal 
diversity,  but  we  differ  from  him  as  to  the  way  in  which 
that  diversity  is  brought  about.  Be  that  as  it  may,  great 
credit  is  due  Darwin  for  being  the  first  to  see  it  was  not 
the  mere  act  of  crossing  which  induced  vigor  but  the  union 
of  different  germinal  complexes.  This  he  states  clearly 
in  the  following  sentences  ('*  Cross  and  Self -Fertilization 
in  the  Vegetable  Kingdom,''  p.  269) :  *'A  cross  between 
plants  that  have  been  self -fertilized  during  several  suc- 
cessive generations  and  kept  all  the  time  under  nearly 


INBREEDING  EXPERIMENTS  117 

uniform  conditions,  does  not  benefit  the  offsprin^r  in  the 
least  or  only  in  a  very  slight  degree.  Mimulus  and  the 
descendants  of  Ipomea,  named  Hero,  offer  instances  of 
this  rule.  Again,  plants  self-fertilized  during  several 
generations  profit  only  to  a  small  extent  by  a  cross  with 
intercrossed  plants  of  the  same  stock  (as  in  the  case  of 
Dianthus),  in;  comparison  with  the  effects  of  a  cross  by 
a  fresh  stock.  Plants  of  the  same  stock  intercrossed  dur- 
ing several  generations  (as  with  Petunia)  were  inferior 
in  a  marked  manner  in  fertility  to  those  derived  from 
the  corresponding  self-fertilized  pknits  crossed  by  a  fresh 
stock.  Lastly,  certain  plants  which  are  regularly  inter- 
crossed by  insects  in  a  state  of  nature,  and  which  were 
artificially  crossed  in  each  succeeding  generation  in  the 
course  of  my  experiments,  so  that  they  can  never  or  most 
rarely  have  suffered  any  evil  from  self-fertilization  (as 
with  Eschscholtzia  and  Ipomea),  nevertheless  profited 
greatly  by  a  cross  with  a  fresh  stock.  These  several  cases 
taken  together  show  us  in  the  clearest  manner  that  it  is 
not  the  mere  crossing  of  any  two  individuals  which  is 
beneficial  to  the  offspring.  The  benefit  thus  derived  de- 
pends on  the  plants  which  are  united  differing  in  some 
manner,  and  there  can  hardly  be  a  doubt  that  it  is  in  the 
constitution  or  nature  of  the  sexual  elements.  Anyhow, 
it  is  certain  that  the  differences  are  not  of  an  external 
nature,  for  two  plants  which  resemble  each  other  as 
closely  as  the  individuals  of  the  same  species  ever  do, 
profit  in  the  plainest  manner  when  intercrossed,  if  their 
progenitors  have  been  exposed  during  several  genera- 
tions to  different  conditions.'^ 

Unfortunately,  in  Darwin's  time  the  key  to  the  sola 


118         INBREEDING  AND  OUTBREEDING 

tion  of  the  problem  of  inbreeding  was  lacking.  Mendel's 
work  was  yet  unrecognized ;  the  principles  of  inheritance 
of  separate  characters,  of  segregation,  of  chance  recom- 
bination, Darwin  was  not  permitted  to  know.  Had  he 
realized  the  way  in  which  recessive  characters  can  be  con- 
cealed for  many  generations  without  making  their  ap- 
pearance until  homozygosity  was  brought  about  by  in- 
breeding, doubtless  his  views  on  the  subject  would  have 
been  materially  changed. 

As  we  have  just  indicated,  and  as  we  shall  have  occa- 
sion to  emphasize  again,  the  greatest  advance  in  our 
knowledge  of  the  significance  of  inbreeding  has  come 
through  linking  its  effects  with  Mendelian  phenomena. 
The  first  experiments  on  the  subject  made  in  the  light 
of  this  discovery  were  those  of  G.  H.  ShuU  and  of  East, 
undertaken  independently  in  1905  with  maize,  an  ideal 
cross-fertilized  species,  as  the  subject. 

Shuirs  investigations  were  not  begun  with  the  object 
of  studying  the  effects  of  self-fertilization,  but  the  studies 
having  involved  parallel  cultures  of  cross-pollinated  and 
seff-pollinated  lines,  it  was  impossible  not  to  have  noticed 
the  smaller  stalks  and  ears  and  the  greater  susceptibility 
to  attacks  of  the  corn-smut  (Ustilago  maydis)  shown  by 
the  latter.  Interest  thus  aroused,  data  were  collected 
bearing  on  the  subject  of  inbreeding,  and  in  1908  his  first 
conclusions  on  the  subject  were  published. 

His  observation  that  the  progeny  of  every  self-fer- 
tilized maize  plant  is  inferior  in  size,  vigor  and  productive- 
ness to  the  progeny  of  a  normal  cross-bred  plant  derived 
from  the  same  source,  corroborated  preceding  investi- 
gations   made    by    Morrow    and    Gardner  ^^2'  ^^^    and 


INBREEDING  EXPERIMENTS  1 1 9 

Sliamel  ^^^ ;  but  the  conclusion  whicli  lie  drew  was  now. 
The  universality  of  this  decrease  in  vigor  was  to  Shall  a 
proof  that  the  injurious  effect  of  inbreeding  could  not  be 
due  to  an  accumulation  of  deficiencies  possessed  by  the 
parents  since  superior  and  inferior  parents  yielded  sim- 
ilar results.  Further,  Shull  noted  that  this  decrease  in  size 
and  vigor  accompanying  self-fertilization,  instead  of 
proceeding  at  a  steady  or  even  at  an  increasinir  rate  as 
might  be  expected  from  this  older  view,  actually  became 
less  and  less  in  succeeding  generations — ])resiiinably  in- 
dicating an  approach  to  stability.  The  neatness  with 
which  these  observations  fit  a  ^Nfendelian  interpretation  of 
inbreeding  did  not  escape  notice.  It  was  pointed  out  how 
one  might  consider  a  com  field  to  be  a  collection  of  com- 
plex hybrids  whose  elementary  components  may  lie 
separated  by  self-fertilization  through  the  operation 
of  the  fundamental  Mendelian  laws  of  segregation 
and  recombination. 

"With  this  working  hypothesis  the  investigations  were 
continued  for  several  years,  papers  on  the  subject  ap- 
pearing in  1909,  1910  and  1911.  Evidence  of  the  hybrid 
nature  of  ordinary  commercial  maize  plants  and  their  de- 
pendence upon  hybridity  for  their  vigor  was  found  in  the 
decided  differences  in  definite,  hereditary,  niorpholoia- 
cal  characters  exhibited  by  self-fertilized  families  having 
a  common  origin,  but  a  further  proof  of  the  validity  of  the 
hypothesis  came  in  testing  the  conclusions  to  which  the 
view  leads.  Obviouslv  crosses  between  ])lants  of  a  single 
family,  which  by  long-continued  self-fertilization  has  be- 
come homozygous  in  nearly  all  its  characters,  should  show 
little  increase  in  vigor  over  self-fertilization;  but  crosses 


120 


INBEEEDING  AND  OUTBREEDING 


between  distinct  self -fertilized  lines  should  often  result 
in  high-yielding  F^  generations  possessing  great  vigor 
and  showing  a  high  degree  of  uniformity.  Again,  crosses 
between  different  near-homozygous  strains,  though  uni- 
form and  vigorous  in  the  Fj  generations,  should  become 
much  more  variable  and  much  less  vigorous  in  the  F2 
generation.  These  general  propositions  Shull  tested  in 
a  limited  way  in  1910  after  his  families  had  been  self-fer- 
tilized for  ^ve  generations.  The  variability  of  two  such 
strains  and  the  crosses  between  them  for  a  definite  and 
easily  determined  character — number  of  rows  per  ear — 
is  shown  in  the  following  table : 


Strain 

Mean 

Coefficient  of  variation 

A 
B 

AXB  (FO 
BXA  (Fi) 
AXB  (FO 
B  X A  (FO 

8.30±.06 
14.10±.15 
12.71±.15 
11.77±.07 
11.84±.ll 
13.79±.ll 

8 .  50  per  cent.  ±  .  47  per  cent. 

9.66  per  cent.  ±  .74  per  cent. 
10.00  per  cent.  ±  .87  per  cent. 

8. 13  per  cent.  ±  .42  per  cent. 
14.64  per  cent,  i  .67  per  cent. 
10.62  per  cent.  ±  .56  per  cent. 

Clearly  the  F^  generation,  made  with  either  type  as  the 
mother,  is  as  uniform  as  the  parent  strains,  but  the  F2 
generations  are  both  more  variable. 

To  test  the  other  corollaries,  nine  different  self-fer- 
tilized families  of  the  fifth  generation  were  compared 
with  families  obtained  by  crossing  two  plants  belonging  to 
each  family;  seven  families  were  raised  as  first-genera- 
tion hybrids  between  these  different  selfed  strains;  ten 
crosses  between  F^  individuals  were  compared  with  ten 
self-fertilizations  in  the  same  families ;  and  ten  families 
were  grown  in  which  self-fertilization  had  been  precluded 
for  five  years.    The  average  height  in  decimeters,  number 


INBREEDING  EXPERIMENTS 


121 


of  rows  per  ear,  and  yield  in  bushels  per  acre  of  these 
fifty-five  families  are  g-iven  in  the  following  table: 


Selfed 

Fclfod 
X  Kibs 

Fi 

F. 

F, 

Holfed 

Fi  Sib 
CroMes 

Cro»- 
breds 

Average  height .  . 
Average  No.  rows 
Average  yield 

19.28 

12.28 
29.04 

20.00 
13.20 
30.17 

25.00 
14.41 
68.07 

23.42 
13.07 
44 .  02 

23.55 
13.02 
41.77 

23.30 
13.73 
47.40 

22 .  05 
15.13 
01.52 

The  sister-brother  (sib)  crosses  give  a  slightly  greater 
height,  number  of  rows  per  ear  and  yield  per  acre  than 
the  corresponding  self-fertilized  families,  an  indication, 
as  Shull  states,  of  some  heterozygosis  still  remaining  in 
the  selfed  families ;  in  other  particulars  Mendclian  expec- 
tation is  wholly  confirmed. 

The  experiments  of  Shull  on  the  effect  of  inbreeding  in 
maize  were  continued  only  from  1905  to  1911.  We  may  be 
pardoned,  therefore,  if  we  describe  the  experiments  be- 
gun in  1905  by  East  at  the  Connecticut  Agricultural  Ex- 
periment Station  in  somewhat  greater  detail,  for  they  are 
still  being  carried  on  by  Jones.  In  fact,  in  point  of  num- 
bers and  scope  they  are  the  most  extensive  experiments 
on  the  problem  of  inbreeding.  The  general  method  of 
procedure  has  been  merely  to  self-pollinate  individual 
plants  from  different  varieties  of  all  the  principal  types 
of  maize.  The  seed  from  such  self-fertilized  plants  has 
been  grown  and  some  plants  again  self-fertilized.  Thus 
a  selfed  plant  has  been  the  parent  of  each  population. 
Over  thirty  different  varieties,  with  several  lines  in 
each  variety,  have  been  inbred  in  this  way.  The  old- 
est strains  have  now  been  self-fertilized  for  twelve 
consecutive  generations. 

In  every  case  there  has  been  a  reduction  in  size  of 
plant  and  yield  of  grain.    Besides  this  result,  to  which 


122         INBREEDING  AND  OUTBREEDING 

there  has  been  no  exception,  the  several  inbred  lines 
originating  from  the  same  variety  have  become  more  or 
less  strikingly  differentiated  in  morphological  characters. 
Some  of  the  differences  which  characterize  the  several 
inbred  strains  in  various  combinations  are  as  follows : 

Colored  and  colorless  pericarps,  cobs,  silks  and  glumes. 
Profusely  branched  tassels  and  scantily  branched  or  unbranched 

tassels. 
Long  ears  and  short  ears. 
Round  cobs  and  flat  cob®. 
Narrow  silks  and  broad  silks. 
Ears  with  various  numbers  of  rows. 
Ears  with  straight  rows  and  ears  with  irregular  rows. 
Ears  with  large  seeds  and  ears  with  small  seeds. 
Ears  high  on  the  stalk  and  ears  low  on  the  stalk. 
Stalks  with  many  tillers  and  stalks  with  few  tillers. 
Leaves  with  straight  margin  and  leaves  with  wavy  margin. 

Many  other  character  differences  governed  by  definite 
inherited  factors  have  been  observed,  but  these  may  serve 
as  illustrations. 

Along  with  these  normal  differences  a  number  of 
characters  have  appeared  which  might  well  be  called  mon- 
strosities, using  the  term  not  because  of  any  abnormality 
in  the  method  of  their  inheritance,  but  because  they  are 
not  fitted  to  struggle  for  place  either  in  agriculture  or  in 
nature.  A  common  occurrence  is  the  isolation  of  dwarf 
plants  which  are  rarely  capable  of  producing  seed  from 
their  own  pollen.  Plants  manifesting  various  degrees  of 
chlorophyll  deficiency  are  also  frequently  found.  This 
may  show  in  the  form  of  an  entire  lack  of  chlorophyll,  as 
seen  in  pure  albino  plants  which  live  only  until  the  supply 
of  food  in  the  seed  is  exhausted;  or,  it  may  appear  as  a 
yellowish  green,  the  plants  struggling  through  to  seed 


INBREEDING  EXPERBIENTS  123 

production— though  with  some  diniciilty.  Some  phuita 
are  obtained  with  ear  malformations  and  thus  produce 
but  a  minimum  amount  of  seed.  Other  phmts  hick  brace 
roots  and  are  unable  to  stand  upright.  Still  others  show 
vaiious  grades  of  pollen  and  ovule  abortion,  and  suscepti- 
bility to  disease. 

The  variability  of  the  inbred  lines  in  respect  to  the 
above  characters  decreased  as  inbreeding  was  continued. 
After  four  generations  they  were  practically  constant  for 
the  grosser  characters.  From  the  eighth  generation  on 
they  have  been  remarkably  uniforai  in  all  characters. 

Inbreeding  the  naturally  cross-pollinated  maize  plant, 
then,  has  these  results: 

1.  There  is  a  reduction  in  size  of  plant  and  in  produc- 
tiveness which  continues  only  to  a  certain  point  and  is  in 
no  sense  an  actual  degeneration. 

2.  There  is  an  isolation  of  subvarieties  differing  in 
morphological  characters  accompanying  the  reduction 
in  growth. 

3.  As  these  subvarieties  become  more  constant  in  their 
characters  the  reduction  in  growth  ceases  to  be  noticc^able. 

4.  Individuals  are  obtained  with  such  characters 
that  they  cannot  be  reproduced  or,  if  so,  only  with 
extreme  difficulty. 

A  large  amount  of  data  has  been  obtained  upon  which 
to  base  these  statements,  but  since  most  of  them  have  been 
published  it  seems  desirable  to  include  only  a  few  illus- 
trations here.  The  strains  which  have  been  the  longest 
inbred  will  serve  to  show  something  as  to  the  effect  which 
inbreeding  has  had  upon  yield  of  grain,  lieight  of  ])lant 
and  other  maize  characters. 

The  original  experiment  began  ^nth  four  individual 


124 


INBEEEDING  AND  OUTBEEEDING 


plants  obtained  from  seed  of  a  commercial  variety  grown 
in  Illinois  known  as  Learning  Dent.  This  variety  was 
given  the  number  1,  and  four  plants  which  were  self- 
pollinated  and  selected  for  continuation  of  the  inbreeding 
experiment  were  numbered  1-6,  1-7,  1-9,  and  1-12.  These 
four  lines  were  perpetuated  each  year  by  self-pollination 
and  will  be  referred  to  hereafter  as  the  Leaming  strains. 
In  the  second  inbred  generation  two  self -pollinated  plants 
in  the  1-7  line  were  saved  for  seed  and  from  them  two 
inbred  lines  were  split  off  which  consequently  came  origi- 
nally from  one  line  inbred  two  generations.  These  were 
numbered  1-7-1-1  and  1-7-1-2.  Many  other  inbred  strains 
coming  from  different  material  have  been  started  from 
time  to  time  and  several  of  them  are  still  being  continued. 
There  is  no  need  to  mention  them  specifically,  except  as 
they  bring  out  special  features. 

TABLE  III 
The  Effect  of  Inbreeding  on  the  Yield  and  Height  of  Maize 


Year 
grown 


1916 
1905 
1906 
1908 
1909 
1910 
1911 
1912 
1913 
1914 
1915 
1916 
1917 


No.  of 
genera- 
tions 
selfed 


Four  inbred  strains  derived  from  a  variety  of  Leaming  dent  corn 


0 
0 

1 

2 
3 
4 
5 
6 
7 
8 
9 
10 
11 


1-6-1-3-etc. 


Yield 

bu.  per 

acre 


74.7 
88.0 
59.1 
95.2 
57.9 
80.0 
27.7 


41.8 
78.8 
25.5 
32.8 
46.2 


Height 
inches 


117.3 


l-7-l-l-etc. 


Yield 
bu.    per 

acre 


86.7 


96.0 

"  97.7 
103.7 


74.7 
88.0 
60.9 

1908460 

63.2 
25.4 


Height 
inches 


1-7-1-2-etc. 


1-9-1-2-etc. 


117.3 


39.4 
47.2 
24.8 
32.7 
42.3 


81.1 


83.5 


84.9 
78.6 


Yield 

bu.  per 

acre 


74.7 
88.0 

60.9 
190759.3 

1908597 

68.1 
41.3 


Height 
inches 


117.3 


90.5 


58.5 


19.2 
37.6 


88.0 


86.9 
83.8 


Yield     1 

bu.  per 

Height 
inches 

acre 

74.7 

117.3 

88.0 

42.3 

51.7 

35.4 

47.7 

26.0 

76.5 

191338.9 

1914454 

85.0 

191621.6 

1  "630.6 

78.7 

191^31.8 

82.4 

INBREEDING  EXPERIMENTS  125 

In  Table  III  the  yield  of  grain  and  heiglit  of  plant  of 
the  four  inbred  Learning  strains  are  given  in  the  suc- 
cessive generations  of  self-fertilization.  In  191G  seed  of 
the  original  variety,  which  had  been  gro\vn  in  the  mean- 
time in  the  locality  in  Illinois  from  whence  it  was  originally 
secured,  was  obtained  and  grown  for  comparison  with  the 
inbred  strains.  This  variety  in  Illinois  in  1905  yielded  at 
the  rate  of  88  bushels  of  shelled  grain  per  acre  and  in 
Connecticut  in  1916  at  the  rate  of  75  bushels.  There  is  no 
reason  for  supposing  that  the  variety  had  changed  to  any 
great  extent  in  the  intervening  years.  Coming  from  Illi- 
nois, it  was  placed  at  a  disadvantage  as  compared  to  the 
inbred  strains,  because  it  was  not  adapted  to  the  local 
conditions,  while  the  inbred  strains,  grown  for  several 
years,  had  been  selected  more  or  less  unconsciously  to 
meet  the  prevailing  conditions.  Even  with  this  in  favor 
of  the  inbred  strains  they  yielded  only  from  one-third  to 
one-half  as  much  as  the  original  variety  grown  under  the 
same  conditions. 

With  regard  to  the  rate  of  reduction  in  yield  or  the 
constancy  of  the  varieties  during  the  later  generations, 
it  is  difficult  to  draw  conclusions  from  these  figures,  owing 
to  the  fluctuation  in  yield  from  year  to  year  due  to  sea- 
sonal conditions  and  to  the  difficulty  of  accurate  testing  in 
field  plot  work,  a  fact  recognized  by  all  who  have  made 
such  tests.  No  yields  for  any  of  the  strains  were  taken 
in  1912.  The  yields  for  1909  and  1915  were  too  low  on 
account  of  poor  seasons.  The  yields  in  1914  were  too  high 
for  the  opposite  reason.  In  1915  the.  yields  were  unreliable 
because  only  a  few  plants  were  available  for  calculation, 
most  of  the  plants  having  been  used  for  hand  pollinations. 


126         INBREEDING  AND  OUTBREEDING 

In  1916  and  1917  the  inbred  strains  were  grown  in  some- 
what larger  plots  and  the  yields  are  fairly  reliable. 

With  these  points  in  mind,  an  examination  of  the  table 
shows  that  from  the  beginning  of  the  experiment  to  the 
ninth  generation  there  has  been  a  tremendous  drop  in 
productiveness,  so  that  in  that  generation  the  strains  were 
approximately  only  one-third  as  productive  as  the  variety 
before  inbreeding.  From  the  ninth  to  the  eleventh  gen- 
eration there  has  been  no  reduction  in  yield  and  prac- 
tically no  change  in  visible  characters.  Height  of  plant, 
as  far  as  the  available  figures  show,  followed  the  same 
course.  The  reduction  which  has  taken  place  occurred  in 
the  first  eight  generations ;  after  that  there  has  been  no 
appreciable  change. 

All  along  the  several  Learning  strains  have  shown 
considerable  difiFerences  in  productiveness  and  in  height. 
Strain  No.  1-6  has  given  the  largest  yields  and  the  tallest 
plants.  It  gave  nearly  50  per  cent,  larger  yields  than  the 
poorest  yielding  strain  in  the  eleventh  year,  and  was  about 
30  per  cent,  higher  than  the  shortest  strain. 

One  of  the  strains,  No.  1-12,  was  lost  in  the  sixth  gen- 
eration. Previous  to  this  time  it  had  been  the  poorest  of 
the  five.  It  was  partially  sterile,  never  produced  seed  at 
the  tip  of  the  ear  and  was  perpetuated  only  with  care. 
Since  the  difficulty  of  carrying  along  any  inbred  strain  is 
great,  owing  to  failure  to  pollinate  at  the  correct  time,  to 
attacks  of  fungus  on  the  ears  enclosed  in  paper  bags,  and 
to  poor  germination  in  the  cold,  wet  weather  common  in 
New  England  at  com-planting  time,  the  loss  of  this  strain 
might  be  easily  accounted  for  without  assuming  continu- 
ous deterioration.  The  strain  probably  could  have  been 
retained  if  sufficient  effort  had  been  put  forth;  but  in 


INBREEDING  EXPERIMENTS  127 

view  of  the  further  reduction  in  other  strains,  it  would 
have  been  extremely  diliicult.  {Since  plants  are  frequently 
produced  which  cannot  be  perpetuated,  however,  it  is 
to  be  expected  that  some  strains  will  also  be  found 
which  cannot  survive.  This  is  good  evidence  that  strains, 
differing  markedly  in  their  ability  to  grow,  are  isolated 
by  inbreeding. 

Plants  of  the  surviving  strains,  while  smaller  in  size 
and  lower  in  productiveness,  are  perfectly  healthy  and 
functionally  normal  in  every  way  except  that  in  many  of 
them  there  is  an  extreme  reduction  in  the  amount  of  pollen 
produced.  These  infertile  types  are  dependent  on  other 
plants  for  pollen  in  order  to  make  the  yields  they  show 
in  open  field  culture ;  when  grown  by  themselves  the  yield 
is  less  due  to  an  inadequate  supply  of  pollen.  On  the  other 
hand,  this  extreme  reduction  in  pollen  production  is  not 
shown  by  all  the  strains,  some  inbred  strains  producing 
pollen  abundantly. 

Prom  the  data  given  in  Table  III  there  is  considerable 
evidence  that  these  plants  have  reached  about  the  limit 
of  their  reduction  in  size  and  productiveness  and  tluit 
whatever  changes  have  taken  place  in  the  last  three  years 
have  been  slight.  Further  inbreeding  is  necessary  for 
one  to  be  positive  on  this  point.  But  as  the  crosses  within 
these  inbred  strains  have  given  no  significant  increases 
over  the  selfed  lines,  and  as  there  has  been  no  visible 
change  in  morphological  characters,  in  the  past  three 
years  at  least,  it  seems  apparent  tliat  the  reduction  in 
vegetative  vigor  and  productiveness  is  ver>'  nearly,  if  not 
quite,  at  an  end. 

Reduction  is  shown  by  inbred  maize  plants  in  other 
characters.    Length  of  ear,  as  well  as  height  of  plant  and 


128         INBREEDING  AND  OUTBREEDING 

yield  of  grain,  is  smaller.  There  is  also  a  slight  reduction 
in  number  of  nodes  and  in  rows  of  grain,  but  in  contrast 
to  the  other  three  characters  the  change  is  almost  negli- 
gible. The  last  two  are  only  slightly  affected  by  environ- 
mental factors  as  compared  with  the  others.  A  plant  may 
be  reduced  to  one-half  its  normal  height  by  being  grown 
in  a  poor  situation,  but  the  number  of  nodes  will  be  nearly 
the  same  in  the  two  cases.  Hence,  we  see  that  inbreeding 
affects  plants  much  in  the  same  way  as  poor  environmen- 
tal conditions. 

In  all  of  the  characters  mentioned  there  is  a  reduction 
in  variability  and  change  in  mean  differing  in  the  several 
lines.  This  is  illustrated  in  Table  IV,  in  which  are  given 
the  data  for  number  of  rows  of  grain  on  the  ear  of  four 
different  plots  of  the  origiual  non-inbred  variety  and 
four  strains  derived  from  this  variety  after  ten  genera- 
tions of  self-fertilization.  The  marked  reduction  in  vari- 
ability is  apparent  both  in  the  restricted  range  of  the 
distribution  of  the  inbred  lines  compared  to  the  variety, 
and  in  the  coefficients  of  variability. 

This  reduction  in  variability  applies  only  to  each  in- 
bred line  separately.  If  all  the  different  lines  were  com- 
bined together  into  one  population  the  variation  would 
be  greater  than  that  shown  by  the  original  material.  This 
is  readily  apparent  from  the  table ;  it  also  follows  from 
the  fact  that  many  characters  are  produced  by  inbreed- 
ing which  are  seldom  seen  in  the  regularly  cross- 
pollinated  stock.  Inbreeding  reduces  variability  within 
separate  lines,  but  increases  variability  in  the  descend- 
ants as  a  whole. 

From  the  curves  on  inbreeding  given  in  the  preceding 
chapter  (Fig.  24),  it  was  seen  that  the  production  of  com- 


INBREEDING  EXPERIMENTS 


129 


I 

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H-^f 


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a  a  d  o 
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to  to 
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t    I 

CO  CO 

I    I 
to  to 
I    I 


I    I    I    I 


I    I    I 


t-t>-i  t~A^^        OOCOCOM         ^-.f? 


II  \    T 


I      I      I      I 
t-*  1-^  (-*  »— • 

4^  »i^  ik.  »^ 

I      I      I      I 

J  J    '    I 

d  C;i  Cn  en 


4k.  4^  »L  4ik 
4^  4^  4k.  4k. 

4»    *k.    4k- 


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to  to 


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■      I      I      I 


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to  Cn  CT>  Cn  Cn 
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(-«  H-*         »-»  H-»         to  to  to  to         t-"  t-i  »-»  l-» 
Cncn       CnOi       Ot--»tOt—       4^.4^.0105 


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130 


INBREEDINa  A^D  OUTBREEDING 


pletely  homozygous  types  by  self-fertilization  is  greatest 
in  the  generations  from  the  third  to  the  sixth  if  a  large 
number  of  factor  differences  are  involved  at  the  start. 
The  experimental  results  obtained  from  these  inbred 
strains  of  maize  fit  this  theory  well.  It  is  not  until  after 
about  three  generations  of  self-fertilization  that  extreme 
types  begin  to  appear.  While  there  has  been  a  reduction 
in  size  and  productiveness  before  this,  it  is  at  this  time, 
or  during  the  next  two  or  three  generations,  that  the 
greatest  diversity  of  types  occurs.  It  is  here  that  most 
of  the  monstrosities  and  plants  which  are  unable  to  re- 
produce themselves  appear. 

From  Table  IV  we  see  that  equally  striking  changes 
in  the  mean  row  number  also  take  place.  The  averages 
have  been  shifted  both  up  and  down  from  the  original 
conditions.  The  greatest  segregation  has  taken  place 
between  the  first  and  the  eighth  generations.  In  the  eighth 
generation  the  lines  were  again  split  up,  but  show  no 
marked  change  after  this  point.  Differences  ia  the  ears 
of  these  inbred  strains  of  com  are  shown  in  Fig.  29. 

The  rate  of  reduction  in  variability  and  rate  of  change 
of  mean  are  shown  by  the  data  for  row  number  of  two 
of  the  inbred  strains  for  successive  years  in  Table  V  and 
Fig.  30.  These  two  Hues  are  descended  from  the  same 
plant  in  the  second  year  of  self-fertilization.  The  figures 
previous  to  the  third  year  are  not  available,  and  in  that 
year  only  for  one  of  the  strains,  but  since  then  a  marked 
change  in  average  row  number,  and  a  reduction  in  vari- 
ability have  taken  place  without  conscious  selection  one 
way  or  the  other.  Though  the  number  of  plants  grown  in 
the  generations  from  the  7th  to  the  10th  are  too  few  to  be 
a  basis  for  accurate  conclusions,  the  sharp  increase  in 


";AS<i'> 


^v 


:-y^ 


y . 


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m 


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Fio.   29. — Representative  samples  of  inbred  strains  of  maize  after   11   tifncratmn.s  <i|  htlf. 
fertilization  showing  cliaracteristie  ciifTereuces  but  uniformity  within  each  »train. 


INBREEDING  EXPERIMENTS 


131 


CO  -X) 


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CnCn 


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4:^  4*.      coco      to  to      '--'-'      oo      ctj  y: 


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132 


INBREEDINa  AND  OUTBREEDING 


average  row  number  and  decrease  in  variability  in  the 
eighth  generation  are  probably  due  to  the  favorable  grow- 
ing conditions  of  that  year — ^witness  the  high  yields  for 
the  inbred  strains  in  that  year  as  given  in  Table  III.  The 
apparent  rise  in  variability  after  the  eighth  generation 
is  in  part  due  to  the  fact  that  the  ears  had  become  some- 


4» 
C 
V 

■H 
O 

t 

%4 


tt) 

n 


20  , 


15  . 


U 
o 

I 

o 

O 

rt  C  o 


Ave.   No.   RoVS 
of  1-7-1-1 


Ave .   No .   Rows 
of  1-7-1-2 


Ave.C.V. 


— T— 

z 


4  S  6  7  8  9  10         U 


Generations  Intred 

Fia.  30. — Graphs  showing  reduction  of  variability  and  segregation   of  ear  row  number 

in  selfed  strains  of  maize. 


what  more  irregular  in  row  number,  so  that  accurate  de- 
termination of  number  of  rows  has  been  more  difficult  in 
the  later  generations.  However,  this  rise  is  more  appar- 
ent than  real  as  the  values  for  the  coefficients  of  variabil- 
ity in  the  intermediate  generations  are  probably  lower 
than  they  would  have  been  if  an  adequate  number  of 
plants  had  been  grown. 

The  effect  of  inbreeding  upon  variability  is  even  more 


INBREEDING  EXPERIMENTS  133 

apparent  in  details  of  plant  and  oar  structure  which  are 
difficult  of  statistical  expression.  The  beautiful  uni- 
foimity  of  these  plants  in  all  characteristics  at  the  pres- 
ent time  is  one  of  their  most  striking  features.  This  can 
be  seen  fairly  well  for  the  ear  chanicters  in  accompany- 
ing illustrations  (Eig.  29).  In  the  minutiie  of  the  tassels, 
leaves  and  stalks  they  show  the  same  striking  uniformity. 
These  minor  details  which  characterize  each  of  these 
groups  of  plants  are  difficult  to  describe  adequat(^ly,  but 
are  perhaps  the  most  noticeable  feature  about  them. 
The  tassels  or  the  ears  of  all  these  four  Learning 
strains,  if  mixed  together,  could  be  separated  without 
the  slightest  difficulty. 

Some  characters  appear  so  rarely  in  plants  they  have 
been  generally  considered  to  be  due  to  what  might  be 
called  physiological  accidents  rather  than  to  inheritance. 
An  illustration  of  this  kind  is  furnished  in  maize  by  the 
occurrence  of  doubled  or  connate  seeds.  Instead  of  one 
embryo  enclosed  in  a  pericarp,  separate  embryos  and 
endosperms  are  present,  with  the  seeds  arranged  back 
to  back  and  the  embryos  facing  in  opposite  directions. 
A  few  seeds  of  this  kind  have  been  described  from  time 
to  time,  but  never  more  than  one  or  two  on  an  occasional 
ear.  From  twelve  inbred  strains  of  a  variety  of  maize 
other  than  the  ones  previously  described,  two  linos  have 
been  obtained  which  produce  these  peculiar  seeds  as  a 
common  feature.  One  of  the  strains  show^s  from  one  to 
six  or  more  on  practically  every  ear.  The  second  strain 
shows  them  more  rarely  and  the  other  ten  strains  derived 
from  the  same  variety  have  never  been  obsor\'ed  to  boar 
them.  Here,  then,  is  a  character  which  does  not  appear 
except  at  rare  intervals  when  the  plants  are  crossed 


134         INBREEDING  AND  OUTBREEDING 

and  in  full  vigor.  When  the  plants  are  brought  to 
homozygosity  and  the  vigor  of  the  plants  is  reduced,  the 
doubled  seeds  appear  in  abundance  in  some  lines,  but  not 
in  all.  A  character,  then,  may  be  governed  in  its  ex- 
pression by  other  characters  and  modified  by  the  vigor 
of  the  plant,  but  in  the  final  analysis  it  is  dependent  upon 
definitely  inherited  factors. 

In  the  same  way  such  indefinite  and  complex  charac- 
ters as  susceptibility  and  resistance  to  disease  are  shown 
to  be  capable  of  segregation.  In  1917  one  of  the  inbred 
Learning  strains  had  not  a  siugle  plant  affected  by  the 
smut  fungus,  although  1000  plants  were  grown  in  differ- 
ent places.  Other  strains  derived  from  the  same  variety 
and  grown  side  by  side  with  the  susceptible  race  showed 
from  5  to  10  per  cent,  of  plants  infected.  Susceptibility 
of  maize  to  smut  thus  seems  to  be  dependent  upon  in- 
herited factors.  As  the  result  of  inbreeding,  these  fac- 
tors may  be  segregated  tato  some  lines  and  not  into  others. 

Although  there  has  been  a  striking  reduction  in  size  of 
plant,  general  vegetative  vigor  and  productiveness,  and 
in  comparison  with  non-iribred  varieties  the  inbred  plants 
are  more  dififtcult  to  grow,  emphasis  must  be  put  upon  the 
fact  that  they  are  normal  and  healthy.  No  actual  degen- 
eration has  occurred.  The  monstrosities  which  are  com- 
mon in  every  field  of  maize,  such  as  the  occurrence  of 
seeds  in  the  tassels,  anthers  in  the  ears,  dwarf  plants, 
completely  sterile  plants,  and  other  similar  anomalies,  now 
no  longer  appear  in  these  inbred  strains.  These  facts, 
taken  together,  should  be  sufficient  to  demonstrate  beyond 
doubt  that  by  far  the  greatest  amount  of  the  general  vari- 
ability found  among  ordinary  cross-fertilized  plants  is 
due  to  the  segregation  and  recombination  of  definite  and 


INBREEDING  EXPERIMENTS  135 

constant  hereditary  factors.  Some  of  the  characters 
which  appear  after  long-continued  inbreeding  are  seldom 
seen  in  continually  cross-pollinated  plants,  and  never  are 
so  many  seen  in  combination.  This  is  because  they  are 
recessive  in  nature  and  complex  in  mode  of  inheritance 
The  most  significant  feature  about  the  cliaracters  wliirh 
make  their  appearance  in  inbred  plants  is  that  none  of 
them  can  be  attributed  directly  to  a  loss  of  a  physiological 
stimulation,  although  undoubtedly  many  of  them  may  be 
modified  by  the  vigor  of  the  plants  upon  which  they  are 
borne.  There  is  no  one  specific  feature  common  to  all 
inbred  strains,  but  simply  a  general  loss  of  vigor,  a  gen- 
eral reduction  in  size  and  productiveness  accompanied  by 
specific  characters  more  or  less  unfavorable  to  the 
plant's  best  development.  But  these  unfavorable  char- 
acters are  never  all  found  in  one  inbred  strain,  nor  is  any 
one  of  them  found  in  all  inbred  strains. 

Although  no  systematic  selection  has  been  practiccnl 
throughout  these  inbreeding  experiments,  a  great  deal  of 
selection  upon  many  characters  has  been  unavoidable  as 
is  the  case  in  any  inbreeding  experiment.  In  maize,  the 
difficulties  of  hand  pollination  result  in  the  selection  of 
plants  whose  staminate  and  pistillate  parts  are  matured 
synchronously.  Any  great  difference  in  this  respect,  par- 
ticularly towards  protandry,  renders  self-fertilization 
difficult  or  impossible  as  the  pollen  is  viable  but  a  short 
time.  Of  course,  all  plants  wiiich  are  weak,  steril«\  dis- 
eased or  in  any  way  abnormal,  tend  to  become  eliminated 
wherever  these  causes  reduce  the  chance  of  obtainincr  seed. 
This  unconscious  selection  becomes  more  ricrid  in  the  lat«^r 
generations  of  inbreedin.s:  as  reduction  in  vicror  and  pro- 
ductiveness becomes  more  pronounced.    Acrain,  the  small 


136         INBREEDING  AND  OUTBREEDING 

amount  of  seed  produced  by  hand  pollination  under  the 
most  favorable  circumstances,  necessitates  the  using  of 
the  best  ears  obtained  for  planting  in  order  to  have 
enough  plants  upon  which  to  make  any  fair  observations. 
These  factors  tend  to  prevent  the  attainment  of  com- 
plete homozygosity.  Nevertheless,  all  the  evidence  at  hand 
indicates  that  the  four  strains  of  Leaming  com  which  have 
been  continuously  self-fertilized  for  twelve  generations 
are  now  very  nearly,  if  not  completely,  homozygous  in  all 
inherited  characters.   As  stated  before,  this  evidence  com- 
prises cessation  of  reduction  in  size  and  productiveness, 
of  reduction  in  variability,  and  of  change  of  average  row 
number  and  other  characters.    But  there  are  still  other 
ways  of  testing  the  proposition.    On  the  theory  that  in- 
crease in  growth  results  from  crossing  when  the  individu- 
als united  differ  in  respect  to  some  inherited  qualities,  if 
no  increase  results,  then  the  parents  have  no  differences. 
These  strains  have  been  tested  in  this  way  by  crossing 
different  plants  within  a  strain  and  comparing  the  crossed 
plants  with  self  ed  plants.  While  some  increases  in  growth 
resulted  from  such  crossing  they  were  balanced  by  de- 
creases in  other  cases,  so  that  the  inconsistencies  are  most 
likely  due  to  difficulty  in  securing  an  accurate  test.    At 
the  same  time  one  should  not  shut  his  eyes  to  the  possi- 
bility that  some  of  the  strains  have  reached  complete 
homozygosity,  while  others,  as  yet,  have  not ;  although  no 
sure  evidence  of  such  a  state  of  affairs  has  been  obtained. 
Most  of  the  direct  experimentation  to  determine  the 
effects  of  inbreeding  has  been  with  cultivated  plants  and 
domestic  animals.  The  question  will  undoubtedly  be  asked, 
therefore,  as  to  whether  the  results  would  have  been  the 
same  had  wild  species  been  investigated.    It  would  be 


INBREEDING  EXPERIMENTS  137 

futile  to  maintain  that  there  is  every  reason  to  supjwse 
wild  species  should  behave  exactly  as  their  domestic 
cousins.  Wild  types,  in  general,  nii.irht  not  present  such 
an  appearance  of  injur>^  under  inbreeding  as  is  often 
sho^vn  by  cultivated  species.  This  would  not  be  due  to 
differences  in  their  method  of  inheritance,  however,  but 
because  wild  species  are  usually  exposed  to  a  more  rigor- 
ous struggle  for  existence  and  tlic  indivi(Uials  are,  there- 
fore, less  likely  to  differ  by  a  large  number  of  hereditary 
factors.  For  such  reason  one  should  expect  experiments 
on  different  wild  species  to  give  rather  varied  results,  and 
in  the  comparatively  small  number  which  have  ])e('n  made 
this  is  the  case.  Castle's  experiments  on  the  fniit  fly  pave 
no  markedly  unfavorable  results.  Collins  states  that  self- 
fertilizing  teosinte,  a  semi-wild  relative  of  maize,  causes 
no  loss  of  vigor.  Yet  Darwin  compared  self-fertilizod  and 
intercrossed  plants  of  several  species  which  are  largely 
cross-fertilized  in  the  wild  with  great  disadvantage  to 
the  former. 

This  discussion  of  the  effects  of  artificial  inbreeding 
in  certain  plants  and  animals  has  been  given  in  some  de- 
tail in  order  to  bring  out  the  many  important  considera- 
tions involved.  There  has  even  been  repetition  in  order 
to  emphasize  the  most  important  points.  Details  are 
merely  by  way  of  parenthesis,  however.  Let  us  now  get 
out  of  the  parenthesis  and  into  the  main  ar^irument. 

From  the  preceding  ohservatioyi  it  can  he  said  that 
inbreeding  has  hut  one  demonstrable  effect  on  organisms 
subjected  to  its  actioii—the  isolation  of  homozygnu^ 
types.  The  diversity  of  the  resultmg  types  depends  di- 
rectly upon  number  of  heterozygous  hereditary  factors 
present  in  the  individuals  with  which  the  process  is  he- 


138         INBEEEDING  AND  OUTBEEEDING 

gun;  it  is  likely,  therefore,  to  vary  directly  with  the 
amount  of  cross-breeding  experienced  by  their  immediate 
ancestors.  The  rapidity  of  the  isolation  of  homozygous 
types  is  a  function  of  the  intensity  of  the  inbreeding. 

Take  the  case  of  maize  as  an  example.  Maize  is  one 
of  the  most  variable  of  cultivated  plants,  and  is  usually 
cross-pollinated  under  natural  conditions.  In  other 
words,  the  individuals  making  up  any  commercial  variety 
of  maize  are  each  and  every  one  heterozygous  for  a  large 
number  of  hereditary  factors — a  heterozygosis  that  is 
kept  up  by  continual  crossing  and  recrossing.  "When  such 
a  variety  is  inbred  there  is  automatic  isolation  of  homo- 
zygous combinations,  following  simple  mathematical  laws 
as  we  have  already  seen.  If  self-fertilization  is  practiced, 
stabilization  through  an  approximately  complete  homo- 
zygosis  occurs  after  a  relatively  small  number  of  genera- 
tions ;  if  a  less  intense  system  of  inbreeding  is  followed, 
the  result  is  the  same,  but  it  is  obtained  more  slowly.  Dur- 
ing thisf  process,  before  stabilization  is  reached,  there  is 
reduction  in  size,  vigor  and  productiveness  following 
somewhat  roughly  the  reduction  in  per  cent,  of  hetero- 
zygousness.  We  can  think  of  this  reduction  in  vigor  as  a 
change  correlated  with  approaching  homozygosis  if  we 
wish,  although  as  we  shall  see  there  is  reason  to  believe  it 
to  be  a  result  of  linked  inheritance.  What  does  occur  is 
a  reduction  in  vigor  of  the  population  as  a  whole  in  each 
generation  associated  with  the  isolation  of  individuals 
more  homozygous  than  their  parents.  Any  particular  in- 
dividual may  be  vigorous  or  weak,  fertile  or  sterile,  nor- 
mal or  monstrous,  good,  bad  or  indifferent,  depending 
wholly  upon  the  combination  of  characters  received, 
many  of  the  characters  which  become  homozygous  will  be 


INBREEDING  EXPERIMENTS  139 

recessives  or  combinations  of  recessives  which  seldom  are 
seen  under  ordinary  circmnstances,  because  they  are  hid- 
den by  their  dominant  allelomorphs.  These  recessives 
are  the  ''corrupt  i'mit"  which  give  the  bad  name  to 
inbreeding,  for  they  are  often— very  often— undesir- 
able characteristics. 

The  homozygous  inbred  strains  after  stability  has 
been  reached  are  quite  comparable  to  naturally  self -fer- 
tilizing species  provided  they  have  passed  as  rigorous 
selection  as  the  latter  have  had  to  undergo  by  reason  of 
natural  competition.  And  Darwin,  as  well  as  others, 
found  that  artificial  self-pollination  causes  no  reduction  in 
such  genera  as  Nicotiana,  Pisum  and  Phaseolus  where 
self-fertilization  is  the  general  rule. 

Are  then  the  immediate  results  of  inbreeding  some- 
times injurious?  In  naturally  cross-fertilized  organisms 
they  most  emphatically  are — nay,  more,  even  disastrous — 
when  we  recall  the  reduction  to  over  half  or  one-third  in 
production  in  grain  and  a  corresponding  decrease  in  size 
of  plant  and  rate  of  growth  in  maize.  But  maize  is  prob- 
ably an  extreme  case.  With  other  organisms  the  results 
are  not  so  bad,  and  in  some  cases,  especially  when  selec- 
tion has  been  made,  no  evil  effects  are  apj)arent.  In  fact, 
there  may  be  an  actual  improvement.  But  the  truth  is,  we 
did  not  set  out  to  answer  that  question.  It  had  already 
received  a  correct  answer.  What  ive  undertook  to  inquire 
was  whether  inbreeding  is  inj^irious  merely  by  reason  of 
the  consangmnity.  We  answer,  No/  The  only  injury 
proceeding  from  inbreeding  comes  from  the  inlieritance 
received.  The  constitution  of  the  individuals  result ins< 
from  a  process  of  inbreedine:  depends  upon  tlio  chance 
allotment  of  characters  preexisting  in  the  stock  before  in- 


140         INBREEDING  AND  OUTBREEDING 

breeding  was  commenced.  If  undesirable  cbaracters  are 
shown  after  inbreeding,  it  is  only  because  they  already 
existed  in  the  stock  and  were  able  to  persist  for  genera- 
tions under  the  protection  of  more  favorable  characters 
which  dominated  them  and  kept  them  from  sight.  The 
powerful  hand  of  natural  selection  was  thus  stayed  until 
inbreeding  tore  aside  the  mask  and  the  unfavorable  char- 
acters were  shown  up  in  all  their  weakness,  to  stand  or 
fall  on  their  own  merits. 

If  evil  is  brought  to  light,  inbreeding  is  no  more  to  be 
blamed  than  the  detective  who  unearths  a  crime.  Instead 
of  being  condemned  it  should  be  commended.  After  con- 
tinued inbreeding  a  cross-bred  stock  has  been  purified  and 
rid  of  abnormalities,  monstrosities,  and  serious  weak- 
nesses of  all  kinds.  Only  those  characters  can  remain 
which  either  are  favorable  or  at  least  are  not  definitely 
harmful  to  the  organism.  Those  characters  which  have 
survived  this  **day  of  judgment''  can  now  be  estimated 
according  to  their  true  worth.  As  we  shall  see  later 
vigor  can  be  immediately  regained  by  crossing.  Not  only 
is  the  full  vigor  of  the  original  steck  restored,  but  it  may 
even  be  increased,  due  to  the  elimination  of  many  unfav- 
orable characters.  If  this  increased  vigor  can  be  utilized 
in  the  first  generation,  or  if  it  can  be  fixed  so  that  it  is 
not  lost  in  succeeding  generations,  then  inbreeding  is  not 
only  not  injurious  but  is  highly  beneficial.  As  an  actual 
means  of  plant  and  animal  improvement,  therefore,  it 
should  be  given  its  rightful  valuation. 


CHAPTER  VII 

HYBKID  VIGOR  OR  HETEROSIS 

Whether  or  not  inbreeding  in  a  race  of  plants  or  ani- 
mals results  injuriously  depends  primarily,  as  we  have 
attempted  to  show,  upon  the  hereditary  constitution  of 
the  organism.  The  beneficial  effect  of  crossing,  heterosis, 
is  a  more  widespread  phenomenon.  It  may  be  expected 
when  almost  all  somewhat  nearly  rehited  foniis  are 
crossed  together.  Even  plants  or  animals  which  show  no 
harmful  results  of  inbreeding  are  frequently  improved 
thus  in  a  remarkable  way.  Moreover,  this  stimulating 
effect  is  immediately  apparent  in  the  individuals  result- 
ing from  the  cross.    It  is  then  at  its  maximum. 

It  is  natural,  therefore,  that  the  early  writers  on  the 
subject  should  have  noticed  and  emphasized  the  good  to 
be  derived  from  crossing  rather  than  the  bad  which  some- 
times results  from  inbreeding.  Almost  without  exception 
the  great  horticultural  writers  of  the  late  eighteenth  and 
early  nineteenth  centuries  noted  the  occurrence  of  hybrid 
vigor,  and  many  of  them  described  it  in  great  detail. 
Among  them  may  be  mentioned  Kolreuter  (1763),  Knight 
(1799),  Mauz  (1825),  Sageret  (1826),  Borthollot  (1827), 
Wiegmann  (1828),  Herbert  (1837),  Lecoq  (1845),  Giirt- 
ner  (1849).  In  fact,  in  Focke^s  compilation  of  this  early 
work,  **Die  Pflanzen-Mischlinge*'  (1881),  cases  of  heter- 
osis worthy  of  special  mention  w^ere  found  in  fifty-nine 
families  of  the  flowering  plants  as  well  as  in  the  conifers 
and  the  ferns.    Animal  husbandmen  were  somewhat  loss 

141 


142         INBREEDING  AND  OUTBREEDING 

inclined  to  acknowledge  and  discuss  tlie  matter,  although 
they  had  an  excellent  example  before  them  in  the  mule — 
an  animal  known  and  appreciated  for  over  four  thousand 
years.  But  the  necessity  of  their  following  the  custom  of 
maintaining  breeds  true  to  certain  fixed  standards  prob- 
ably accounts  for  their  conservatism  in  estimating  the 
importance  of  the  phenomenon. 

K61reuter,i25  the  first  botanist  to  study  artificial 
plant  hybrids,  made  many  interspecific  crosses  in  the 
genera  Nicotiana,  Dianthus,  Verbascum,  Mirabilis,  Da- 
tura and  others,  which  astonished  their  producer  by  their 
greater  size,  increased  number  of  flowers  and  general 
vegetative  vigor,  as  compared  with  the  parental  species 
entering  into  the  cross.  He  -gives  many  exact  measure- 
ments of  his  hybrids  and  speaks  with  some  awe  of  their 
** statura  portentosa'*  and  '^ambitus  vastissimus  ac  alti- 
tudo  valde  conspicua,"  Later,  after  some  observations 
on  certain  structural  adaptations  for  cross-pollination 
which  he  interpreted  correctly,  he  made  a  passing  re- 
mark which  plainly  showed  he  thought  Nature  had 
intended  plants  to  be  cross-fertilized  and  that  benefit 
ensued  therefrom. 

Some  forty  years  after,  Thomas  Andrew  Knight,^22  ^ 
horticulturist  who  was  a  very  keen  observer,  noticed  sim- 
ilar instances  of  high  vigor  in  his  crosses :  in  the  descrip- 
tion of  these  experiments  we  note  the  following  remarks 
concerning  a  cross  between  two  varieties  of  peas : 

By  introducing  the  farina  of  the  largest  and  most  luxuriant  kinds 
into  the  blossoms  of  the  most  diminutive  and  by  reversing  the  process 
I  found  that  the  powers  of  the  male  and  female  in  their  effects  on  the 
offspring  are  exactly  equal.  The  vigor  of  the  growth,  the  size  of  the 
seeds  produced,  and  the  season  of  maturity,  were  the  same,  though  the 
one  was  a  very  early,  and  the  other  a  very  late  variety.    I  had,  in  this 


HYBRID  VIGOR  OR  HETEROSIS  143 

experiment,  a  striking  instance  of  the  stimulating  effects  of  cro6tiing  the 
breeds;  for  the  smallest  variety,  whose  height  rarely  exceeded  two  feet, 
was  increased  to  six  feet,  whilst  the  height  of  the  large  and  liLxunaul 
kind  was  very  little  diminLshed. 

It  is  evideut  that  iu  tiiLii  particular  cajse  Krnglil  was 
dealing  with  dwarf  and  standard  peas,  and  dominance  of 
the  tali  standard  habit  of  growth  is  to  be  expected.  This 
is  not  the  correct  interpretation  of  the  majority  of  his  ob- 
servations on  hybrid  vigor,  however;  a  sulhcient  number 
of  really  striking  manifestations  of  the  phenomenon  were 
found  to  give  adequate  foundation  for  his  anti-inbreeding 
principle,  elaborated  by  Darwin  lifty  years  later. 

Probably  the  most  extensive  series  of  early  experi- 
ments on  hybridization  were  those  of  Gartner."^^  This 
enthusiastic  worker  crossed,  or  attempted  to  cross,  every- 
thing available  to  him.  According  to  Lindley,^-**  he  made 
10,000  pollinations  between  700  species,  and  produced  1250 
different  hybrids.  Many  of  his  attempted  crosses  either 
failed  to  produce  seed,  or  if  seed  was  produced,  gave  feeble 
plants;  but  a  great  number  of  the  hybrids,  where  the 
crosses  were  made  between  plants  not  too  distantly  re- 
lated, showed  distinct  evidence  of  hybrid  vigor  mani- 
fested in  many  different  ways.  Gartner  speaks  especially 
of  their  general  vegetative  luxuriance,  increase  in  root 
development,  height,  number  of  flowers,  the  facility  of 
their  vegetative  propagation,  their  hardiness  and  early 
and  prolonged  blooming.    He  says : 

One  of  the  most  conspicnons  and  common  eliaracteristics  of  plant 
hybrids  is  the  luxuriance  of  all  their  parts,  a  luxuriance  that  is  shown  in 
the  rankness  of  their  growth  and  a  prodisral  development  of  root  shoota, 
branches,  leaves,  and  blossoms  that  could  not  be  induced  in  the  parent 
stocks  by  the  most  careful  cultivation.  The  hybrids  usually  reach  the 
full  development   of  their  parts  only  when   plante<l   in    the  open,   as 


144         INBREEDING  AND  OUTBREEDING 

Kolreuter  (125)  has  already  remarked;  when  grown  in  pots  and  thus 
limited  in  food  supply  their  tendency  is  toward  fruit  development  and 
seed  production. 

Besides  possessing  general  vegetative  vigor,  hybrids  are  often 
noticeable  for  the  extraordinary  length  of  their  stems.  In  various 
hybrids  of  the  genus  Verbascum,  for  example  lychnitis-thapsus,  the  stem 
shoots  up  12  to  15  feet  high,  with  a  panicle  7  to  9  feet,  the  six  highest 
side  branches  2  to  3  feet,  and  the  stem  1  1/4  inches  in  diameter  at  the 
base:  in  Althaea  cannahino-officinalis  the  stem  is  10  to  12  feet;  in 
Malva  mauritano-sylvestris  9  to  11  feet;  in  Digitalis  purpureo-ochroleuca 
8  to  10  feet,  with^anicles  4  to  5  feet;  and  in  Petunia  nyctaginifloro- 
phoenicea  and  Lobelia  cardinali-syphilitica  3  to  4  feet  each.  Prof. 
Wiegmann  also  corroborates  these  observations. 

The  root  system  and  the  power  of  germination  of  hybrids  are 
highly  correlated  with  their  great  vegetative  vigor.  Many  hybrids,  there- 
fore, which  are  not  so  luxuriant  in  growth  as  those  just  described,  for 
example  Dianthus,  Lavatera,  Lycimn,  Lychnis,  Lobelia,  Geum,  and 
Pentstemon  hybrids,  put  forth  stalks  easily  and  therefore  are  readily 
propagated  by  layers,  stolons,  or  cuttings.  The  observa4;ions  of  Kolreuter 
(125),  and  of  Sageret  (191)  agree  with  ours  in  this  respect. 

Luxuriation  expresses  itself  at  times  as  proliferation;  for  instance, 
in  Lychnis  diurno-ftos  cuculi  the  receptaculum  is  changed  to  a  bud  that 
puts  forth  branches  and  leaves.  If,  moreover,,  the  vigor  of  the  hybrids 
especially  affects  the  stem  and  the  branches,  particularly  their  length, 
nevertheless  the  leaves  take  part  in  it  by  becoming  larger.  Hybrids  in 
the  genera  Datura,  Nicotiana,  Tropaeolum,  Verbascum,  and  Pentstemon 
are  examples. 

Naudin,^^^  the  contemporary  of  Mendel,  whose  ideas 
very  nearly  resembled  modern  conception  of  heredity, 
likewise  gives  many  excellent  illustrations  of  hybrid  vigor 
from  interspecific  crosses  which  he  made  in  Papaver,  Mir- 
abilis,  Primula,  Datura,  Nicotiana,  Petunia,  Digitalis, 
Linaria,  Luffa,  Coccinea  and  Cucumis.  Out  of  35  crosses 
within  these  genera  24  show  positive  evidence  of  heter- 
osis. The  cross  of  Datura  Stramonium  with  D,  Tatula 
was  particularly  notable  in  this  respect.  Both  reciprocal 
hybrids  were  twice  as  tall  as  either  parent. 


HYBRID  VIGOR  OR  HETEROSIS  145 

Even  MendePs  classic  pea  hybrids  supply  further  in- 
stances of  increase  in  size  resulting  from  crossing.  Con- 
cerning them,  he  says : 

The  longer  of  the  two  paxental  stems  is  lUiually  exceeded  by  the 
hybrid,  a  fact  which  is  possibly  only  attributable  to  the  greater  hixiLruince 
which  appears  in  all  parts  of  the  plants  when  stems  of  vei*y  different 
lengths  ai"e  crossed.  Thus,  for  instance,  iii  repeated  experiiiients,  stema 
of  1  foot  and  of  b  feet  in  length  yielded  without  exception  hybrids  which 
varied  in  length  between  G  feet  and  7  1/2  feet. 

i^'ocke,'*^  in  the  book  already  cited,  gives  the  results  of 
a  series  of  experiments  nearly  as  extensive  as  those  of 
Gartner  and  catalogoies  his  own  results  along  with  those 
of  his  predecessors.  The  compilation  is  so  careful,  so 
painstaking,  and  so  complete  that  one  may  tui'n  to  the 
linal  conclusions  of  the  author  without  fear  of  error  as 
far  as  the  facts  are  concerned.  He  says:  *'  Crosses  be- 
tween different  races  and  different  varieties  are  distin- 
guished from  individuals  of  the  pure  type,  as  a  rule,  by 
their  vegetative  vigor.  Hybrids  between  markedly  dif- 
ferent species  are  frequently  quite  delicate,  especially 
when  young,  so  that  the  seedlings  are  difficult  to  raise. 
Hybrids  between  species  or  between  races  that  are  more 
nearly  related  are,  as  a  rule,  however,  uncommonly  tall 
and  robust,  as  is  sho^\Ti  by  their  size,  rapidity  of  growth, 
earliness  of  flowering,  abundance  of  blossoms,  long  dura- 
tion of  life,  ease  of  asexual  propagation,  increased  size  of 
individual  organs,  and  similar  characters.'' 

The  attention  of  these  earlier  hybridizers  was  mainly 
directed  towards  interspecific  crosses,  ])ut  they  also  noted 
a  great  number  of  instances  in  which  crosses  between 
closely  related  forms,  such  as  varieties  or  sub-variotios 
of    cultivated    plants,    gave   remarkable   increments    in 

10 


146         INBREEDING  AND  OUTBREEDING 

growth.  In  fact,  we  have  found  no  record  of  intervarietal 
crosses  where  delicate  or  weak  progeny  resulted.  It 
would  not  be  useful,  however,  to  attempt  to  canvass  the 
literature  for  all  those  cases  in  which  crossing  either  did 
or  did  not  result  to  the  advantage  of  the  offspring.  A  list 
of  the  crosses  would  alone  fill  a  volume.  It  is  only  neces- 
sary to  point  out  that  the  value  to  be  derived  from  cross- 
ing thus  made  so  evident  gave  great  impetus  to  the  study 
of  tioral  structures  as  adaptations  for  cross-pollination. 
So  zealously  was  this  line  of  investigation  pursued,  that 
knowledge  of  the  methods  of  pollination  in  the  angio- 
sperms  soon  exceeded  that  of  any  other  phase  of  general 
botany.  The  interpretation  placed  upon  many  of  these 
floral  mechanisms  was  fantastic,  to  say  the  least,  the  en- 
thusiastic claims  of  the  workers  rivalling  those  of  zool- 
ogists in  mimicry  and  protective  coloration.  The  net  re- 
sult was  simply  to  show  how  widespread  were  means  of 
cross-pollination.  It  might  be  said  to  have  proved  that 
cross-fertilization  is  an  advantage ;  it  did  not  prove  it  to 
be  indispensable.  There  were  too  many  naturally  self- 
fertilized  plants  for  any  such  conclusion. 

Of  all  the  work  on  the  effects  of  crossing  in  pre-Men- 
delian  times,  that  of  Darwin  is  the  most  important.  With 
it  we  get  a  new  insight  into  the  meaning  of  inbreeding  and 
outbreeding.  Darwin  was  the  first  to  see  it  was  not  the 
mere  act  of  crossing  which  was  beneficial.  He  satisfied 
himself  on  this  point  by  crossing  different  flowers  on  the 
same  plant  and  different  plants  of  similar  strains.  In 
neither  case  was  there  any  positive  evidence  of  an  effect. 
But  crosses  between  different  varieties  or  species  of 
plants  gave  unmistakable  signs  of  invigoration.  In  24 
cases  out  of  37,  cross-fertilization  increased  the  height 


HYBRID  VIGOR  OR  HETEROSIS  147 

of  plant;  in  5  out  of  7  experiments,  the  weight  was  in- 
creased. Moreover,  the  crossed  phints  frequently  flow- 
ered earlier  and  in  many  other  ways  showed  their  advan- 
tage over  the  parent  races. 

Darwin  extended  his  observations  to  the  animal  king 
dom  and  his  views  on  the  whole  subject  are  summed  up 
concisely  in  the  following  paragraph  from  ''Animals  and 
Plants  under  Domestication ^^  ''The  gain  in  constitu- 
tional vigor  derived  from  an  occasional  cross  between  in- 
dividuals of  the  same  variety,  but  belonging  to  differ  on  t 
families,  or  between  distinct  varieties,  has  not  been  so 
largely  or  so  frequently  discussed  as  have  the  evil  effects 
of  too  close  interbreeding.  But  the  former  point  is  the 
more  important  of  the  two,  inasmuch  as  the  evidence  is 
more  decisive.  The  evil  results  from  close  interbreeding 
are  difficult  to  detect,  for  they  accumulate  slowly  and 
differ  much  in  degree  with  different  species,  whilst  the 
good  effects  which  almost  invariably  follow  a  cross  aro 
from  the  first  manifest.  It  should,  however,  be  clearly 
understood  that  the  advantage  of  close  interbreeding,  as 
far  as  the  retention  of  character  is  concerned,  is  indis- 
putable and  often  outweighs  the  evil  of  a  slight  loss  of 
constitutional  vigor.'* 

From  this  statement  Darwin  evidentlv  considered  the 
ill  effects  of  inbreeding  and  the  good  effects  of  crossincr  to 
be  two  different  things.  He  was  right  in  stressinc:  the 
benefit  from  crossing  rather  than  the  injury  from  close 
mating,  but  wrong  in  thinking  the  evil  effects  accumulated 
as  inbreeding  was  continued.  Such  a  belief  is  not  substan- 
tiated by  more  recent  experiments,  as  has  been  shown  in 
the  last  chapter.  It  is  true,  however,  that  the  effect  of 
inbreeding  may  not  be  as  noticeable  in  the  first  generation 


148         INBREEDING  AND  OUTBREEDING 

as  the  invigoration  immediately  apparent  after  crossing. 
The  effects  of  outbreeding,  unlike  those  of  inbreed- 
ing, are  shown  both  by  plants  which  are  naturally  self- 
fertilized  and  by  those  which  are  cross-fertilized.    Many 
of  the  illustrations  already  given  are  from  plants  almost 
go  invariably  self -fertilized.    Crossing  within  a  pure  line  of 
^    such  a  species  shows  no  heterosis;  but  if  the  parents 
i  ^    united  in  the  cross  differ  more  or  less  in  minor  external 
a    features   an  increase  in  growth  is   usually  to  be  ex- 


>-} 


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pected.  This  has  been  shown  to  be  true  for  peas,  toma- 
toes, tobacco  and  many  other  normally  self-feirtilized 
^  ,  forms  among  cultivated  plants,  as  well  as  for  several 
^   wild  species. 

An  extensive  series  of  crosses  between  different  Nico- 
tiana  species  has  been  reported  by  East  and  Hayes.^^ 
The  majority  of  these  crosses  were  taller  than  the  average 
of  the  two  parents  and  many  were  taller  and  more  vigor- 
ous than  either  parent.  Some  of  the  crossed  plants  were 
completely  sterile.  In  certain  cases  these  were  weak,  non- 
vigorous  plants,  but  there  were  others  in  which  inability 
to  produce  seed  was  accompanied  by  increased  vigor. 
Thus,  while  occasionally  the  increased  development  of 
sterile  hybrids  may  be  due  in  part  to  their  having  ex- 
pended no  energy  in  seed  production,  the  fact  that  many 
vigorous  hybrids  manifest  greater  ability  to  produce  seed 
shows  this  is  a  relatively  unimportant  factor  and  entirely 
inadequate  to  account  for  the  great  vigor  obtained  where 
there  is  full  fertility. 

A  fair  example  of  the  way  in  which  height  is  gained 
by  crossing  is  given  by  East  and  Hayes, ^^  a  cross  of  Nico- 
tiana  rustica  hrazilia  Comes  and  N.  rustica  scahra  Comes. 


HYBRID  VIGOR  OR  HETEROSIS  149 

The  frequency  distributions  of  height  of  j^huit  of  the  two 
parents  and  the  reciprocal  hybrids  are  given  in  Tabk»  VI. 

TABLE  yi 

Hbiqht  of  Crosses  Between  Nicotiana  Rustica  Scabra  (352)  and 

N.  HusTiCA  Brazilia  (349) 


Variety  or 

Class  centers  in  inobefl 

24 

27    30    33    36    3U    42    45    48    51    54    57    60 

63    66    69    72    76    78 

349 
352 

352X349,  F, 
349X352,  F, 

4 

10  22  14    7 

2     1     5  11  10  17    0 

13    0     5 

3     5 

5    5    r,    1    1 

2     4     0     5     12 

in  both  of  the  first  hybrid  generations  the  average  height 
is  above  the  major  extreme  of  either  parent.  Similar 
increases  in  height  were  obtained  when  a  commercial 
variety  of  tobacco  was  crossed,  first  with  a  variety  from 
the  same  locality,  then  with  one  from  the  opposite  side  of 
the  world  identical  with  the  first  in  external  appearance. 
On  the  other  hand,  strains  of  tobacco  from  seed  grown  in 
Connecticut  when  crossed  with  plants  of  the  same  vari- 
eties from  seed  grown  in  Italy  showed  no  increase  in 
vigor.  Hence,  the  mere  fact  of  residence  in  d liferent 
parts  of  the  world — that  is,  exposure  to  different  environ- 
mental conditions — has  no  necessar>'  relation  to  the 
phenomenon  of  hybrid  vigor,  for  such  individuals  may 
be  alike  in  constitution.  Danvin's  repeated  emphasis  of 
the  good  derived  from  crossing  plants  whose  ancestors 
were  exposed  to  different  conditions  was  because  he 
thought  such  differences  in  environment  brought  about 
germinal  changes.  This  attitude,  therefore,  does  not 
detract  from  his  general  position  tliat  it  is  differences 
in  germinal  construction  which  bring  about  liybrid  vigor; 
and  this  is  the  principal  point  at  issue. 


150         INBREEDING  AND  OUTBREEDING 

Tlie  manifestations  of  heterosis  are  most  noticeable 
as  increases  in  size.  This  gain  in  size  in  plants  which  are 
more  or  less  determinate  in  their  number  of  parts  is  made 
up  of  an  increase  in  the  size  of  parts  rather  than  in  the 
number  of  parts.  In  maize  the  number  of  nodes  is  in- 
creased much  less  in  comparison  to  length  of  internodes. 
For  example,  in  a  large  series  of  crosses  between  inbred 
strains  of  maize  height  of  plant  on  the  average  advanced 
27  per  cent.,  whereas  the  number  of  nodes  rose  only  6 
per  cent.  Corresponding  to  the  increase  in  intemode 
length  there  is  an  extension  in  diameter  of  stalk,  length 
and  breadth  of  leaves.  Root  development  is  proportion- 
ally augmented.  Both  the  tassels  and  ears  are  larger,  and 
frequently  two  ears  develop  on  crossed  plants  where 
either  parent  produces  one,  the  color  of  the  foliage  tes- 
tifying to  the  greater  vigor. 

The  greatly  enhanced  growth  of  a  plant  may  be  made 
up  by  increase  in  the  size  of  cells,  as  well  as  by  a  multi- 
plication in  the  number  of  ceUs.  However,  in  a  cross 
between  different  species  of  Catalpa  no  differences  could 
be  seen  in  tracheid  length,  although  the  cross  was  con- 
siderably taller  and  larger  in  diameter. 

The  principal  effect  of  crossing  maize  is  shown  by  an 
additional  production  of  seed.  A  number  of  crosses  have 
given  180  per  cent,  increases  in  yield  of  grain  over  their 
inbred  parents.  Examples  of  what  can  be  done  are  seen  in 
the  accompanying  illustrations  (Figs.  31  and  32).  Im- 
provement in  yield  is  shown  by  crosses  between  inbred 
strains  derived  originally  from  the  same  variety,  as  well 
as  between  crosses  of  strains  derived  from  different  vari- 
eties or  even  from  quite  distinct  types.  The  results  have 
been  very  wonderful  as  a  whole,  giving  at  the  very  least 


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HYBRID  VIGOR  OR  HETEROSIS  151 

a  return  to  the  condition  of  the  original  stock  before  in- 
breeding was  commenced.  Some  combinations  regularly 
give  greater  increases  than  others,  but  in  every  case  such 
differences  are  small  as  compared  with  those  between  the 
crosses  and  the  inbred  parents. 

Although,  in  the  main,  reciprocal  crosses  give  about 
the  same  result,  some  variation  in  this  respect  is  habitu- 
ally shown.  In  general,  there  is  a  correlation  between  the 
yield  of  the  better  parent  strain  and  the  yield  of  the  cross. 
The  crosses  in  which  strain  No.  l-f)  has  been  used  as  the 
female  parent  have  regularly  given  the  highest  yields,  and 
this  strain  is  the  most  vigorous  and  productive  of  the 
four  inbred  Leaming  strains  used  in  our  illustrations. 

In  a  comparison  of  crosses  between  inbred  strains  of 
maize  with  ordinary  outcrossed  varieties  the  inbred  hy- 
brids are  handicapped  because  they  have  to  start  from 
small,  poorly  developed  seeds.  This  handicap  is  brought 
out  clearly  by  a  comparison  of  second  generation  plants 
grown  from  self -fertilized  seed  produced  on  vigorous 
hybrid  plants,  with  hybrid  plants  grown  from  seed  pro- 
duced on  inbred  plants.  The  first  generation  starts  off 
poorly,  as  shown  in  the  accompanying  illustration  (Fiir. 
33),  but  soon  catches  up  and  passes  the  second  generation. 
At  maturity  the  second  generation  is  shorter  and  loss 
productive,  although  it  has  a  much  greater  variability. 
The  third  generation  from  selfed  plants  of  this  particular 
cross  has  been  grown,  and  there  is  still  further  loss  of  the 
stimulation  which  is  at  its  maximum  in  the  first  gonora- 
tion.  On  continued  inbreeding  these  families  presumably 
would  exhibit  a  continuation  of  the  same  course  of  reduc- 
tion in  size,  vigor  and  variability  shown  in  the  original 
inbreeding  experiment,   until  homoz^-gosify   was   acrain 


152         INBREEDING  AND  OUTBREEDING 


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Growth  Curves  of 
Two  In"bred  Strains 
of  Maize  and  Their 
Hybrids. 


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Numljer  of  Days  from  Planting 


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33. — Graphs  showing  growth  curves  of  two  inbred  strains. of  maize  and  thier  first 
and  eeoond  generation  hybrids. 


HYBRID  VIGOR  OR  HETEROSIS  153 

reached.  The  resulting  inbred  strains  would  have  about 
the  same  amount  of  development  as  the  original  inbred 
strains,  but  would  probably  differ  from  them  in  appear 
ance  through  the  possession  of  diflerent  combinations  of 
characters.  The  principal  point  is  that  the  vigor  and  size 
lost  by  inbreeding  are  immediately  restored  by  crossing, 
but  lost  again  on  further  inbreeding.  It  is  a  transit^)ry 
effect,  for  the  most  part,  impossil)le  of  fixation. 

Increases  in  yield  of  grain  are  also  fre^juently  ob- 
tained when  ordinary  commercial  varieties  of  maize  are 
crossed.  Rarely  are  the  increases  greater  than  10  per 
cent.,  however,  and  even  this  is  more  commonly  to  be 
expected  when  varieties  of  somewhat  different  iypa  are 
used;  for  example,  flint  and  dent.  Most  varieties  of  corn 
are  now  so  widely  crossed  and  furthermore  are  so  near 
the  limit  of  production  that  great  advances  are  not  to  be 
expected.  Collins  ^^  has  obtained  especially  large  incre- 
ments in  >deld  by  hybridizing  typos  of  com  from  different 
geographical  regions.  Three  different  varieties  of  corn 
from  the  southwest — Hopi,  Brownsville  and  Hairy  Mexi- 
can— each  gave  an  increase  of  100  per  cent,  or  more  when 
crossed  with  a  variety  from  China  having  seeds  with  a 
different  type  of  endosperm. 

Even  before  the  plants  are  obtained  there  is  a  striking 
effect  of  crossing  in  an  immediate  increase  in  the  size  of 
seed.  This  was  noted  by  Roberts, ^^''^  and  established  very 
clearly  by  Collins  and  Kempton  ^  through  pollinating 
ears  of  maize  with  a  mixture  of  the  plants  own  pollen  and 
of  a  different  sort.  By  taking  advantage  of  the  phenom- 
enon of  double  or  **  endosperm**  fertilization,  the 
experiment  was  so  designed  that  the  ontrTossed  seeds 
could  be  distinguished  by  differences  in  endosponn  color. 


154         INBEEEDING  AND  OUTBREEDING 

Advances  in  average  weight  of  seed  ranging  from  3  to  21 
per  cent,  were  obtained.  With  inbred  strains  as  parents, 
the  increases  are  even  greater,  ranging  from  5  to  35  per 
cent.  The  seeds  have  a  heavier  embryo  as  well  as  a 
heavier  endosperm,  yet  curiously  enough  they  mature 
faster  than  the  selfed  seeds  on  the  same  ears. 

It  is  a  point  of  some  interest,  perhaps,  that  there  is  no 
selective  action  favoring  the  foreign  pollen  when  these 
pollen  mixtures  are  applied.  This  matter  has  been  deter- 
mined very  carefully  on  account  of  its  bearing  on  Men- 
delian  theory,  but  it  also  answers  in  the  negative  the 
question  of  whether  there  is  an  effect  of  heterosis 
manifested  by  a  selective  chemotropism  before  the  zygote 
is  formed. 

Darwin,  in  ^' Cross  and  Self -Fertilization  in  the  Vege- 
table Kingdom, ''  compares  the  time  of  flowering  of  28 
crosses  between  different  types  of  plants  which  had  shown 
distinct  evidence  of  hybrid  vigor.  Of  them,  81  per  cent, 
flowered  before  the  parents.  In  other  cases,  where  no 
heterosis  was  shown  in  other  characters  there  was  no  ac- 
celeration of  the  blooming  period.  These  results  have 
been  corroborated  in  crosses  between  garden  varieties  of 
tomatoes  and  of  sweet  corn,  where  a  tendency  to  put  for- 
ward the  time  of  both  flowering  and  maturing  has  been 
shown  to  accompany  increases  in  size.  Shortening  the 
time  of  growth  thus  seems  to  be  one  of  the  many  ex- 
pressions of  an  increased  metabolic  efSciency  on  the  part 
of  the  hybrid  plant. 

Increased  longevity,  viability,  endurance  against  un- 
favorable climatic  conditions,  and  resistance  to  disease 
have  also  been  frequently  noted  as  properties  of  hybrids. 
Kolreuter  ^^^  and  Wiegmann  "^^  both  mention  these  points. 


bt 


Fig.   34. — James  River  ^\'alIlUt,  a  faiijnii.s  tree  cuiisidcriMl  to  iu-  a  n 
tween  the  Persian  walnut  and  tlic  coninion  liuttcmnt.     .\c<M»rtlinj;  to  (\  S    ~ 
of  the  .Arnold  Arboretum,  tliis  is  the  most  remarkaMc  hardwood   trrv  in  tht   I  in! 
height   10()  feet,  spread  of  brandies  134  feet,  eircumfe-renre  of  trunk  ^ll  fi-i-t.    1 1 >• 


•r 


HYBRID  VIGOR  OR  HETEROSIS  155 

and  Gartner  "^^  gives  them  bis  especial  attention.  Under 
the  heading,  "Ausdauer  und  Lebenstouacitiit  der  P^as- 
tardpflanzen, ' *  he  makes  the  following  statements: 

There  is  certainly  no  essential  difference  between  annual  and 
biennial  plants  and  between  these  and  perennials  in  regard  to  their 
longevity,  for  frequently  different  individuals  oi'  the  same  species  have 
a  longer  life  at  times  as,  for  example,  Draba  venia  which  has  bolh 
annual  and  biennial  forms.  The  longevity  of  a  plant  thei-eby  f umi^hea 
no  specific  difference  but  at  most  only  signitictj  a  variabihty.  ILowever, 
in  hybrids  this  difference  deserves  special  consideration.  In  mo^t 
hybrids  an  increased  longevity  and  greater  endurance  can  bo  obeerved 
as  compared  to  their  parental  races  even  if  they  come  into  bloom  a 
year  earlier.  The  union  of  an  annual,  herbaceous  female  plant  uith  a 
perennial,  shi-ubby  species  does  not  shorten  the  life  cycle  of  the  forth- 
coming hybrid,  as  the  union  of  Hyoscyanms  agrestis  with  niger,  Nicotiunn 
rustica  "with  perennis,  Calceolaria  plantaginea  with  rugosa  shows.  So 
also  in  reciprocal  crosses  w^hen  the  perennial  species  furnishes  the  seed 
and  the  annual  species  supplies  the  pollen,  as  Nicotiana  glauca  v^ilh 
Langsdorfjiij  Dianthus  caryophyllus  with  chitiensis,  Malva  sylcestns 
with  mauritiana  or  biennials  with  perennials  and  reciprocally,  as 
Digitalis  purpurea  with  ochroleuca  or  lutea,  and  lutea  with  purpurea,  or 
ochroleuca  with  purpurea.  From,  the  union  of  two  races  of  different 
longevity  a  hybrid  usually  results  into  Avhich  the  longer  life  of  one  or 
the  other  of  its  parent  races  is  carried  whether  it  comes  from  the  male 
or  female  parent  species. 

Many  more  instances  are  given  by  Gartner  supporting 
the  conclusion  previously  reached  by  Kolr enter  that  the 
longer  life  of  hybrid  plants  is  to  be  counted  among  their 
usual  properties. 

Gartner  also  gives  several  examples  of  endurance  to 
unfavorable  weather  conditions  by  hybrids.  Many  of  his 
tobacco  hybrids  actually  survived  the  winters  in  the  open 
field  in  south  Germany  when  the  parents  were  killed. 

The  hardiness  of  hybrids  is  frequently  shown  by  a 
great  resistance  to  parasitism.  Gornert'^'^  states  that  teo- 
sinte  and  the  first  generation  cross  of  teosinte  and  maize 


156         INBREEDING  AND  OUTBREEDING 

are  not  attacked  by  the  apMds  which  damage  maize.  The 
cross  between  the  inbred  strain  of  maize  most  susceptible 
to  smut,  previously  mentioned,  and  the  strain  not  affected, 
gives  a  hybrid  which  is  only  slightly  parasitized.  The 
same  thing  has  been  noted  in  crosses  between  other 
strains  of  maize,  some  of  which  are  quite  badly  damaged 
by  an  unidentified  leaf  blight  organism.  Radish  seedlings 
which  were  naturally  cross-fertilized  were  much  less  dam- 
aged by  damping-off  fungus  than  uncrossed  seedlings 
from  the  same  plants.  Resistance  is  not  shown  by  all 
first  generation  hybrids  when  the  parents  diif  er  in  suscep- 
tibility. Some  cases  are  known  in  which  the  hybrids  are 
fully  as  susceptible  as  the  less  immune  parent.  In 
the  majority  of  crosses  reported,  however,  in  which  re- 
sistance to  parasitism  is  a  factor  the  hybrids  tend  to 
show  resistance. 

Among  the  diverse  manifestations  accompanying 
heterozygosity  may  be  mentioned  viability  of  seed.  In 
maize,  crossed  and  selfed  seeds  from  the  same  ears  have 
shown  a  difference  of  16  per  cent,  germination  in  favor 
of  the  crossed  seeds.  The  crossed  seedlings  appeared 
earlier  and  grew  faster  from  the  first. 

Increased  facility  of  vegetative  propagation  of  hy- 
brids was  frequently  noted  by  the  early  hybridizers. 
Sageret  ^^^  makes  particular  note  of  a  hybrid  tobacco 
which  easily  propagated  itself  vegetatively.  Many  of  our 
cultivated  fruits  which  are  propagated  by  buds,  grafts, 
cuttings,  etc.,  owe  part  of  their  excellence  at  least  to  the 
fact  that  they  are  in  a  heterozygous  condition.  Moreover, 
there  is  no  evidence  to  prove  that  plants  lose  any  of  their 
hybrid  vigor  in  long  continued  vegetative  multiplication 
through  innumerable  generations. 


HYBRID  VIGOR  OR  HETEROSIS  157 

In  general,  as  noted  before,  there  is  similarity  between 
the  effect  of  heterozygosis  and  that  of  a  good  environ- 
ment. Those  characters  which  are  quickest  to  be  modi- 
fied by  external  factors  also  show  the  greatest  change 
on  crossing.  A  good  illustration  of  this  is  a  Nicotiana 
cross  which  was  ahove  the  average  of  the  parents  in  botli 
height  and  leaf  size.  The  length  of  the  corolhis,  on  tlio 
other  hand,  a  character  very  slightly  affected  by  the  en- 
vironment, was  not  increased.  There  is  at  least  one 
difference  between  the  two,  however;  in  time  of  maturity, 
environment  and  heterosis  have  somewhat  opposite  ef- 
fects. Generally  speaking,  favorable  growing  con<]iti()ns 
tend  to  delay  flowering  and  maturing,  whereas  conditions 
which  stunt  the  plants  tend,  like  heterosis,  to  hasten  them. 

Each  of  these  effects  is  by  no  means  always  present 
when,  a  cross  is  made.  The  usual  and  of  course  the  most 
noticeable  effect  is  the  increase  in  size.  But  crossing  may 
have  a  stimulating  effect  upon  certain  part^  and  a  de- 
pressing effect  on  others.  This  is  shown  in  many  species 
crosses  in  which  reproductive  ability  is  greatly  roducod 
or  even  totally  eliminated,  while  at  the  same  time  vegeta- 
tive growth  is  enormously  increased.  Freeman  and  Sax 
have  independently  obtained  seeds  from  crosses  between 
common  bread  wheats  and  macaroni  wdieats  which  were 
shrunken  in  appearance  and  small  in  size,  owning  to  a  poor 
development  of  the  endosperm.  The  embryos  were  well 
developed,  however,  and  the  plants  produced  gave  dis- 
tinct evidence  of  hybrid  vigor. 

In  this  discussion  there  has  been  a  noticeable  omission 
of  the  effect  of  crossing  on  animals.  Illustrations  are  not 
lacking  that  crossing  frequently  is  highly  ])onori('ial  to 
animals;  but  animals  do  not  funiish  as  desirable  research 


158         INBEEEDING  AND  OUTBREEDING 

material  for  this  particular  problem  as  plants,  on  account 
of  their  bisexuality,  as  was  explained  earlier,  and  for  this 
reason  but  few  quantitative  data  are  available.  There  is 
no  question  but  that  animals  behave  the  same  as  plants 
in  heredity ;  therefore,  one  might  transfer  the  conclusions 
reached  in  the  one  kingdom  to  the  other  without  apology, 
for  the  effects  of  inbreeding  and  cross-breeding  are 
wholly  and  solely  the  working  out  of  the  laws  of  heredity. 
At  the  same  time,  it  will  not  be  amiss  to  present  some  of 
the  results  obtained  by  zoologists,  for  they  strengthen  the 
case  immensely. 

In  the  two  cultivated  species  of  insects  which  form  our 
sole  instances  of  domestication,  bees  and  silkworms,  there 
seems  to  be  evidence  of  increased  vigor  on  crossing  only 
in  silkworms (Toyama  ^^^),  In  the  fruit  fly,  however,  upon 
which  the  greatest  amount  of  genetic  work  has  been  done, 
Castle,2i  Moenkhaus,^*^  Hyde^^  ^nd  Muller^^*  all  found 
size,  fecundity  and  general  constitutional  vigor  increased 
remarkably,  particularly  when  the  strains  crossed  had 
been  inbred  previously.  In  the  rotifer,  Hydatina,  Whit- 
ney 216  and  A.  F.  Shull  ^®®  obtained  similar  results.  Fur- 
ther, Gerschler^®  describes  and  figures  first  generation 
crosses  between  different  genera  of  fishes  which  show  very 
marked  increases  in  size. 

In  birds  also  there  is  such  an  increase  in  vigor  that 
poultry  fanciers  often  cross  two  distinct  strains  and  sell 
the  progeny  because  of  their  rapid  growth  and  large  size. 
No  attempt  is  made  to  breed  from  the  hybrids ;  they  are 
simply  produced  because  of  their  vigor.  When  very  great 
differences  in  size  exist,  there  is  not,  of  course,  an  increase 
in  size  sufficient  to  throw  the  individual  of  the  first  hybrid 
generation  above  the  larger  parent,  as  is  shown  by  the 


HYBRID  VIGOR  OR  HETEROSIS  159 

work  of  Pliillips  ^•*^  on  crosses  between  the  large  Freiicli 
Rouen  and  the  small  domestic  Mallard  duck,  and  by  the 
work  of  Punnett  ^^^  on  crosses  betwen  the  iSilver  Sebright 
bantam  and  Gold-pencilled  Hamburgh  breeds  of  poultry. 
There  is  an  increase  over  the  average  of  the  two  parents, 
but  the  i*'i's  do  not  reach  the  size  of  the  larger  parent 
race.  Part  of  the  reason  for  the  comparatively  small 
sizes  of  the  i^\'s  in  these  crosses,  however,  is  due  to  the 
fact  that  the  crosses  were  always  made  on  the  i>mall  hens 
allowing  the  hybrid  birds  to  get  their  start  in  life  with 
only  the  nutriment  stored  in  the  smaller  eggs. 

The  greatest  amount  of  data  on  this  subject,  just  aa 
there  is  the  greatest  amount  of  interest,  has  been  obtained 
from  the  mammals.  In  the  meat  breeds  of  cattle,  swine 
and  sheep,  as  in  poultry,  it  is  a  common  practice  to  cross 
distinct  races  and  sell  the  progeny.  The  increase  in  size 
and  the  rapidity  with  which  this  size  is  obUiined  are  so 
general  a  phenomenon  that  it  bids  fair  partially  to  replace 
the  older  method  of  pure  line  breeding.  Not  only  are 
varietal  crosses  thus  characterized,  but  speciiic  crosses. 
We  have  already  mentioned  the  mule.  With  the  disad- 
vantage attached  to  sterility,  the  mule  certainly  would  not 
have  held  its  own  throughout  the  past  forty  centuries 
were  it  not  for  its  tremendous  capacity  for  work  and  its 
remarkable  resistance  to  disease.  Crosses  between  the 
ass  and  the  zebra,  and  between  the  cow  jmd  the  zebu  also 
give  animals  of  considerable  merit,  and  one  can  hardly 
refrain  from  thinking  that  within  a  few  years  some  con- 
siderable use  ^\dll  be  made  of  them. 

For  precise  data  on  the  effect  of  cro.^sing  ditTeront 

II       races,  however,  we  must  turn  to  the  small  maninmls  used 

so  constantly  in  experimental  work,  the  mouse,  the  rat,  the 


160 


INBREEDING  AND  OUTBREEDING 


guinea-pig  and  the  rabbit.  One  need  go  no  further  than 
to  cite  the  work  of  Castle  and  his  students  at  the  Bussey 
Institution  of  Harvard  University,  the  work  of  Miss  King 
at  the  Wistar  Institute  of  Anatomy  and  that  of  Wright 
at  the  Bureau  of  Animal  Industry  of  the  United  States 
Department  of  Agriculture.  The  painstaking  researches 
of  these  investigators  show  mthout  question  that  the 
effect  of  crossing  on  animals  is  the  same  as  upon  plants. 


1 

Weight 
ini 

(f  F,.  Cut  "Band C 

Oran 
fififi 

8 

d^aeeB 

X 

dF,.Ci 

tVBandC 

600 

/ 

^ 

dCullen 

400 
200 

^ 

y 

[^ 

^ 

Age  in  Daya  40 


do 


120 


100. 


200 


240 


sso 


320 


3G0 


40O 


FiQ.  35. — Growth  curves  of  males  of  race  B  guinea-pigs  and  Cavia  cutleri  and  their  Fi 

and   Ft   hybrids.      (After    Castle.) 

The  results  from  one  genus  is  typical  of  them  all. 
Castle  ^^  made  a  cross  between  a  domestic  guinea-pig  and 
a  wild  cavy,  Cavia  cutleri.  The  first  generation  hybrid 
males  weighed  about  85  grams  at  birth,  which  is  slightly 
more  than  the  young  of  either  pure  race,  and  retained  this 
lead  throughout  their  subsequent  life  as  is  shown  by  the 
growth  curve  in  Fig.  35.  At  maturity  they  weighed  about 
890  grams,  as  compared  with  800  grams  for  the  guinea- 
pig  ancestor  and  420  grams  for  the  cutleri  ancestor. 

The  second  generation  hybrids  of  both  sexes  were 


HYBRID  VIGOR  OR  HETEROSIS  ICl 

smaller  than  the  first  generation  hy])ri<]s  from  birth  on, 
showing  that  some  of  the  growtli  impetus  produwHi  by  the 
hybridization  had  not  beun  retained.  13ut  the  growth 
curve  of  the  second  generation  hybrids  rises  rapidly  at 
first,  showing  the  healthy  start  in  life  they  obtained  from 
their  vigorous  F^  mothers. 

Perhaps  no  such  increase  in  vigor  as  that  shown  in  the 
species  cross  just  described  is  usually  found  when  dif- 
ferent sub-races  are  crossed.  It  would  not  be  expecti'd, 
for  ordinary  races  of  mammals  are  contiinially  iMMng 
crossed  within  the  variety  and,  therefore,  hybriilization 
would  not  be  expected  to  increase  heterozygosis  to  any 
marked  degree.  But  results  similar  to  those  obtaineil  in 
plants  may  be  expected  if  the  genetic  conditions  are  sim- 
ilar. This  is  proved  by  the  data  Wright  obtained  when  ho 
crossed  guinea-pigs  bom  of  unrelated  inbred  mothers  and 
fathers.  The  cross-breds  were  distinctly  superior  to  their 
inbred  relatives  in  nearly  all  characters  coimected  with 
vigor.  In  spite  of  the  fact  that  their  inbred  mothers  were 
small  and  somewhat  deficient  in  vigor,  a  slightly  larger 
per  cent,  of  cross-breds  than  of  inbreds  were  bom  alive, 
and  a  distinctly  larger  per  cent,  of  those  born  alive  were 
raised.  They  were  somewhat  heavier  at  l)irtli  in  a  given 
size  of  litter  and  gained  in  weight  much  more  rapidly 
between  birth  and  weaning.  They  matured  earlier  and 
produced  larger  litters  and  produced  them  more  regularly 
than  the  inbreds. 

Thus  the  results  with  animals  are  comparable  to  those 
obtained  with  plants  in  all  essential  features.  Brietly,  in 
crosses  which  are  fertile  the  effects  aro  sueh  as  to  con- 
tribute to  a  greatly  increased  reproductive  ability,  making 
11 


ry.' 


162         INBREEDING  AND  OUTBREEDINa 

possible  a  larger  number  of  offspring.  The  degree  to 
which  heterosis  is  expressed  is  correlated,  within  limits, 
with  the  differences  in  the  uniting  gametes.  When  homo- 
zygous forms  are  crossed,  it  is  at  its  maximum  in  the 
first  hybrid  generation,  and  diminishes  in  subsequent  gen- 
erations of  inbreeding  as  segregation  occurs  and  homo- 
zygosity is  again  attained.  It  is  a  widespread  phenom- 
enon and  accompanies  heterogeneity  of  germinal  consti- 
tution whether  the  organisms  crossed  are  from  the  same 
or  diverse  stocks,  whether  they  have  been  produced  under 
similar  or  under  different  environmental  conditions; 
although  it  is  not  apparent  until  the  zygote  is  formed, 
from  that  time  on  it  is  expressed  in  many  ways  through- 
out the  lifetime  of  the  individual  and  is  undiminished 
by  asexual  propagation. 

These  are  the  effects  of  cross-breeding  upon  develop- 
ment in  which  we  have  been  particularly  interested,  those 
in  which  the  organizations  of  the  combining  gametes  are 
sufficiently  compatible  to  permit  continued  propagation. 
But  it  must  not  be  forgotten  that  we  have  dealt  with  only 
one  part  of  the  problem.  As  the  differences  between  the 
forms  increase  limits  are  reached  beyond  which  the  organ- 
isms neither  reproduce  nor  flourish.  One  can  arrange  a 
series  in  plants  in  which  (1)  the  parents  are  so  diverse 
the  cross  cannot  be  made;  (2)  the  seed  obtained  fails  to 
germinate  under  any  set  of  conditions;  (3)  the  hybrids 
are  so  weak  they  are  unable  to  reach  maturity;  (4)  the 
hybrids  are  extremely  vigorous,  but  sterile  except  pos- 
sibly in  back-crosses ;  (5)  the  hybrids  are  fully  fertile  and 
more  vigorous  than  either  parent;  or  (6)  the  parents  may 
be  so  closely  related  no  effects  whatever  are  to  be  noted. 


HYBRID  VIGOR  OR  II1:TKKoSIS  IG.'i 

A  somewhat  similar  series  can  be  arranged  with  animals, 
although  usuiilly  in  wide  crosses  if  the  hybrids  can  Ix' 
obtained  at  all  they  are  as  large  or  larger  than  the  average 
between  parents.  A  satisfactory  interpretation  of  the 
vigor  of  hybridization  must  take  all  these  facts  into  con- 
sideration, even  though  tliey  may  not  be  the  result  of  the 
operation  of  one  single  law. 


CHAPTER  VIII 
CONCEPTIONS  AS  TO  THE  CAUSE  OF  HYBEID 

viaoR 

The  early  plant  hybridizers,  although  they  frequently 
discussed  the  increased  size  and  vigor  of  their  crosses, 
seldom  commented  on  the  effect  of  inbreeding,  and  made 
no  speculations  as  to  the  cause  of  either.  The  animal 
breeders  of  the  period  were  more  imaginative.  Ac- 
quainted with  both  phenomena,  but  more  familiar  with  the 
results  of  inbreeding,  they  unhesitatingly  linked  the  two 
— the  first  as  an  antidote  for  the  second.  They  attributed 
most  of  the  injurious  effects  which  appeared  in  their  herds 
to  the  concentration  of  undesirable  traits.  If  unfavor- 
able characters  and  tendencies  to  disease  were  present, 
mating  similar  animals  brought  out  these  undesirables 
more  pronouncedly;  whereas,  if  healthy  animals  from  un- 
related herds  were  brought  in,  such  tendencies  were 
checked,  the  defects  disappeared,  and  the  health  and  vigor 
of  the  herds  returned. 

Darwin,  however,  refused  to  ascribe  any  large  part 
of  the  effects  of  inbreeding  to  this  cause.  He  knew  of 
many  cases  in  which  weakened  animals  from  different  in- 
bred herds  had  been  mated  together,  and  gave  progeny 
of  full  health  and  vigor  and  of  increased  size.  The  unde- 
sirable features  induced  in  both  herds  by  inbreeding  dis- 
appeared when  animals  of  the  different  herds  were  mated. 
Instead  of  a  concentration  of  the  less  favorable  traits  of 
the  two  parental  lines  the  reverse  seemed  to  have  oc- 
curred. Similar  cases  in  plants  were  familiar  to  him,  and 

164 


CAUSE  OF  HYBRID  VIGOR  1G5 

proved  beyond  question  tlie  ^a'eat  advantage  to  be  gainLxi 
by  crossing  even  when  the  individuals  themselves  were 
weak.  These  facts,  taken  together  with  the  many  mar- 
velous and  intricate  contrivances  of  phuits  to  insure 
cross-pollination,  led  him  to  believe  that  selt'-fertilizatioii 
was  inherently  harmful  and  something  to  be  avoided  it" 
possible.  The  benefits  accming  Trom  crossing  he  ascrilied, 
as  we  have  seen,  to  the  meeting  of  sexual  elements  liaving 
diverse  constitutions. 

After  Darwin's  contribution  to  the  problem  of  inbree<l- 
ing  no  progress  was  made  until  less  than  two  decades  ago, 
when  the  Mendelian  discovery  opentsl  up  s(j  many  new 
possibilities.  The  conception  of  an  inheritance  made  up 
of  separable  units  aroused  a  new  interest  in  the  matter 
and  made  possible  a  unilied  and  satisfactory  interpreta- 
tion of  all  the  facts. 

Mendel  had  shown  that  characters  from  one  i»arent 
might  disappear  completely  in  the  progeny  only  to  reap- 
pear in  subsequent  generations  in  some  of  the  offspring. 
Surely  here  was  something  of  importanee  to  the  inbreed 
ing  problem.  Unfavorable  charaeters  might  vanish  when 
different  organisms  were  crossed;  but  they  were  merely 
hidden.    Inbreeding  revealed  thom  for  what  they  were. 

Shortly  after  Menders  experiments  became  known. 
Bateson  crossed  two  pure  white  Howered  varieties  of  tlie 
sweet  pea.  Instead  of  having  the  whiti^  flowers  oharac 
teristic  of  two  parental  races,  the  hybrid  llowers  wer*^ 
purple.  The  wild  progenitor  of  tlie  sweet  pea  has  purj*!^' 
flowers.  Here  was  a  case  in  which  crossing  brought  back 
previously  existing  conditions,  a  return  to  the  wild  iy\^o 
characters.  This  phenomenon  had  been  obser\'ed  long 
before  this  time;  in  fact,  it  was  so  well  known  it  had  been 


166         INBEEEDING  AND  OUTBREEDING 

given  a  special  name.  Atavism^  or  the  reappearance  of 
previously  existing  characters,  was  immediately  put  upon 
a  Mendelian  basis  by  hypothecating  two  separable  factors 
both  of  which  were  necessary  for  the  production  of  the 
end  result.  This  was  proved  by  the  fact  that  later  white 
flowered  plants  were  obtained  which  did  not  produce  color 
on  crossing  with  either  of  the  original  white  flowered  va- 
rieties and  therefore  lacked  both  factors.  The  light  in- 
creased. Some  of  the  chaotic  observations  of  the  earlier 
hybridizers  began  to  be  understood  as  orderly  facts. 

Ten  years  after  the  rediscovery  of  MendePs  work  a 
symposium  was  held  by  the  American  Society  of  Natur- 
alists on  the  ' '  Genotype  Hypothesis, ' '  an  indication  of  the 
growing  importance  of  the  ideas  associated  with  the  name 
of  Johannsen.  The  basis  of  the  genotype  conception  is 
that  individuals  which  are  visibly  alike  may  be  germi- 
nally  unlike — merely  an  extension  of  the  above  Mendelian 
concepts.  Johannsen's  contribution  was  the  idea  that  the 
unit  factors  of  Mendel  are  relatively  constant  and  stable 
in  whatever  combinations  they  occur,  and  that  the  vari- 
ability of  a  constantly  cross-fertilized  population  is 
largely  due  to  the  segregation  and  recombination  of  these 
unmodified  factors.  When  such  a  heterogeneous  popula- 
tion is  continuously  self -fertilized,  homozygosity  is  ulti- 
mately attained,  and  much  of  the  previous  variability  dis- 
appears. Similar  individuals  making  a  homozygous,  pure 
breeding  population,  are  known  as  a  pure  line,  and  while 
they  still  vary  as  affected  by  different  environmental  con- 
ditions, such  variability  does  not  respond  to  selection,  and 
the  average  condition  is  not  changed.  Although  there  is 
still  a  question  as  to  the  degree  of  stability  of  the  Mendel- 
ian unit  factor,  as  there  is  to  the  degree  of  stability  of 


CAUSE  OF  HYBRID  VIGOR  ir.? 

the  atom,  the  principle  of  the  pure  line  has  been  finnly 
established  by  an  ever-increasing  body  of  evidence,  and 
is  of  the  utmost  importance  in  a  proper  understanding  of 
the  facts  involved  in  inbreeding  and  outbreeding. 

The  first  application  of  these  principles  to  the  problem 
of  inbreeding  was  made  with  the  results  from  maize  al- 
ready described.  It  was  shown  tliat  self-fertilization 
automatically  brings  about  homozygosity,  and  with  it  a  re- 
duction of  a  great  deal  of  the  variability  commonly  shown. 
Along  with  this  reduction  in  variability,  certain  charac- 
ters manifest  themselves  which  are  more  or  less  unfavor- 
able to  the  plants'  best  development.  Plants  with  sterilo 
tassels  and  sterile  ears  and  plants  which  lack  chorophyll 
appear,  cannot  reproduce  themselves,  and  are  eliminated. 
Other  characters  come  to  light  which  do  not  cause  the 
extinction  of  the  plants  directly,  but  which  more  or  less 
handicap  them  in  their  development;  for  example,  partial 
chlorophyll  deficiency,  dwarfness,  bifurcated  organs,  con- 
torted stems  and  deficient  root  systems. 

All  of  these  characters  are  sho\Mi  in  small  numbers  in 
a  cross-pollinated  field  of  maize,  but  not  in  sufficient  fre- 
quency to  reduce  productiveness  seriously.  As  the  result 
of  self-fertilization  some  of  the  strains  o])tained  possess 
certain  of  these  unfavorable  characters  as  regular  fea- 
tures. Here,  then,  is  an  explanation  of  part  of  the  in- 
jurious effects  of  inbreeding.  Unfavorable  characters 
are  segregated  out,  which  reduce  the  developmental  efTi- 
ciency  of  the  plants  which  possess  them.  lUit  if  unfavor- 
able characters  are  concentrated  in  some  lines,  favorable 
characters  are  concentrated  in  others.  Some  have  more 
of  the  favorable  than  of  the  unfavorable,  hence  some 
strains  resulting  from  inbreeding  are  better  than  others. 


168         INBKEEDING  AND  OUTBREEDING 

But  all  strains  in  maize  are  so  greatly  reduced  by  in- 
breeding that  none  can  be  compared  in  productiveness  to 
the  normal  cross-pollinated  plants.  Something  besides 
ordinary  segregation  must  be  involved  in  this  well-nigh 
universal  effect  of  inbreeding. 

It  was  apparent  that  when  germinal  heterogeneity 
was  at  the  maximum  the  greatest  vigor  was  shown.  When 
this  heterogeneity  was  reduced  by  inbreeding,  vigor  was 
lost.  Hence,  the  fundamental  fact  that  hybrid  vigor 
varies  directly  with  heterozygosity  was  clearly  estab- 
lished. To  account  for  the  greater  vigor  and  increased 
development  of  hybrids,  it  was  only  necessary  to  postu- 
late that  a  developmental  stimulation  was  evolved  when 
different  germ  plasms  were  united.  This  hypothesis 
(East  and  Hayes,^^  ShulP®^)  satisfied  all  the  essential 
facts,  and  for  the  first  time  the  effects  of  inbreeding  and 
cross  breeding  were  clearly  understood  in  their  true  rela- 
tion to  each  other.  Inbreeding  was  not  a  process  of  con- 
tinuous degeneration;  it  was  a  process  of  Mendelian 
segregation,  and  its  effect  was  directly  related  to  the 
number  and  type  of  characters  existing  originally  in  a 
heterozygous  condition.  If  unfavorable  characters  were 
covered  up  by  favorable  characters,  inbreeding  brought 
them  out  whenever  a  simplification  of  the  germ  plasm  al- 
lowed them  to  appear.  Inbreeding  was  in  eif  ect  the  isola- 
tion of  homozygous  hereditary  complexes  from  an  hetero- 
zygous hereditary  complex.  If  the  best  of  these  combina- 
tions failed  to  attain  the  development  of  the  original 
stock,  it  was  thought  to  be  because  they  were  deprived  of 
a  stimulus  which  only  accompanied  heterozygosity  and 
which  seemingly  was  impossible  to  fix. 

This  hypothesis,  by  associating  all  the  facts  of  inbreed- 


CAUSE  OF  HYBRID  VIOOR  ](V^ 

ing  and  outbreeding  with  the  phenomena  of  Mendcliaii 
heredity  was  a  great  step  forward.  It  wont  as  far  as  it 
was  possible  to  go  at  the  time  it  was  devised,  and  it  is 
capable  of  interpreting  all  the  facts  to-day.  ]5ut  it  held 
some  disadvantages.  The  assumption  of  a  physiologiciil 
stimulation  arising  from  the  interactic^n  of  fliffcreni 
hereditary  factors  was  not  altogether  satisfactory,  for  it 
locked  the  door  on  any  hope  of  originating  pure  strains 
having  as  much  vigor  as  first  generation  hybrids.  For- 
tunately, the  development  of  Mendelian  heredity  has  been 
such  that  this  part  of  the  hypothesis  can  be  sui-^orseded. 

The  basis  for  this  hypothetical  stimulation  was  seen  in 
the  fact  that  fertilization  is  usually  necessary  to  start  the 
development  of  the  Q^g.  In  most  cases,  without  the  union 
with  the  sperm,  the  eg^  cannot  divide  and  development 
is  prevented.  The  reaction  of  the  different  substances 
HDrought  together  at  fertilization  stimulates  cell  division 
and  starts  development.  This  made  it  reasonable  to  as- 
sume that  when  the  egg  and  sperm  differed  in  here<iitary 
factors  stimulus  to  development  was  increased  and  con- 
tinued throughout  the  growth  of  the  resulting  organism. 
According  to  the  view  of  G.  IT.  Slinll  and  of  Fast,  it  was 
the  interaction  of  different  elements  in  the  nuclei  that 
produced  the  stimulation.  A.  F.  Shull,^^^  on  the  other 
hand,  assumed  the  stimulation  to  result  from  the  inter- 
action of  the  new  substances  brought  in  by  the  s|)erm  with 
the  maternal  cytoplasm.  In  his  opinion  the  stimulation 
might  persist  for  a  time  even  after  homozygosis  was  at- 
tained, because  foreign  elments  brought  in  by  the  original 
cross  would  still  remain  to  react  with  the  cytoplasmic 
matter.  Moreover,  it  was  further  assumed  that  the  stim- 
ulation might  decrease  in  long-continued  asexual  propa- 


y" 


170         INBREEDING  AND  OUTBREEDING 

gation  through  the  cytoplasm  becoming  adjusted  to  a 
heterozygous  nucleus.  This  theory  was  proposed  as  an 
interpretation  of  a  reduction  in  vigor  which  he  had  found 
in  parthenogenetically  reproduced  rotifers.  The  recent 
facts,  however,  are  more  in  accord  with  the  former  view 
because  (1)  the  stimulus  actually  is  lost  as  homozygosity 
is  attained,  and  (2)  the  evidence  of  vigor  being  reduced 
in  continued  asexual  reproduction  is  not  at  all  conclusive. 

The  reasons  for  holding  the  whole  stimulation  hypoth- 
esis in  abeyance  at  present  has  developed  from  the  follow- 
ing facts.  In  1910,  Keeble  and  Pellew^^^  offered  a  con- 
crete illustration  of  a  purely  Mendelian  method  by  which 
increased  growth  could  result  from  crossing.  They  united 
two  varieties  of  garden  peas,  which,  as  grown  by  them, 
each  ranged  from  5  to  6  feet  in  height.  The  first  genera- 
tion grown  from  this  cross  was  from  7  to  8  feet  in  height, 
2  feet  taller  than  either  parent,  a  result  comparable  to 
heterosis.  The  second  generation  showed  segregation 
into  four  classes,  one  class  containing  plants  as  tall  as  the 
first  generation,  two  classes  having  plants  similar  in 
height  to  the  two  parents,  and  one  class  made  up  of 
dwarfs  shorter  than  either  parent.  The  two  classes  of 
medium  tall  plants,  similar  in  height,  were  differentiated 
in  the  same  manner  as  the  parental  races ;  one  had  thick 
stems  and  short  intemodes,  the  other  had  thin  stems  and 
long  internodes  with  fewer  of  them.  The  number  of 
plants  falling  into  these  four  classes  agreed  closely  with 
the  expectation  for  a  dihybrid  ratio  (9:3:3:1)  where 
two  factors  showing  dominance  are  concerned. 

Keeble  and  Pellew  assumed  two  hereditary  factors  to 
be  involved:  one  producing  thick  stems,  the  other  long 
internodes.    These  factors  they  designated  T  and  L.    One 


CAUSE  OP  HYBRID  VIGOR  171 

of  the  parental  varieties  was  medium  in  height,  because 
it  possessed  one  of  these  factors ;  e.g.,  that  for  thick  stems, 
but  lacked  the  other.  Such  a  plant  had  the  f  onnula  TTil. 
The  other  variety  was  of  medium  height,  because  it  lacked 
this  T  factor,  but  possessed  the  factor  for  long  intemodes, 
and  was  given  the  formula  it  LL.  Both  of  these  factors 
showed  dominance  over  the  aUelomorphic  condition; 
hence,  the  first  generation  of  the  cross  was  taller  than 
either  parent  because  both  factors  were  present.  Whether 
or  not  later  investigations  have  justified  this  precise  in- 
terpretation makes  no  material  difference  in  the  discussion 
here.  Taken  as  it  stands,  it  is  a  beautiful  instance  of  the 
way  in  which  complementary  action  of  dominant  factors 
may  increase  a  character  in  a  first  generation  hybrid  over 
its  expression  in  either  parent. 

The  investigators  attempted  to  generalize  from  this 
experiment  and  to  apply  a  dominance  interpretation  to 
the  many  other  cases  in  which  an  increase  in  growth  is 
occasioned.  As  the  matter  stood  at  that  time,  however, 
it  was  impossible  to  see  why  recombination  of  all  the 
dominant  factors  concerned  in  the  increased  growth  of 
the  first  generation  could  not  readily  be  obtained,  and 
hence  some  individuals  be  produced  having  maximum 
size  and  vigor,  yet  unaffected  by  inbreeding  because 
of  their  homozygous  condition.  In  other  words,  in  gen- 
erations after  the  first  it  ought  to  be  possible  to  obtain 
some  strains  having  all  the  dominant  factors  and  others 
with  all  these  dominant  factors  lacking.  Any  such  race 
could  be  rendered  homozygous ;  thereafter,  self-fertiliza- 
tion would  not  result  in  a  less  vigorous  progeny.  And 
while  such  results  may  have  been  obtained  in  the  peas, 
investigators  have  not  been  able  to  duplicate  them  in  the 


172         INBEEEDING  AND  OUTBEEEDING 

many  other  crosses  which  showed  hybrid  vigor.  Further- 
more, not  only  was  the  union  of  such  simple  factorial  com- 
binations inadequate  to  account  for  the  frequency  of  the 
widespread  occurrence  of  heterosis,  but  there  was  another 
seemingly  insurmountable  objection  to  the  interpreta- 
tion. It  was  pointed  out  that  if  heterosis  were  due  solely 
to  dominance  of  independent  factors,  the  distribution  of 
the  second  generation  would  be  unsymmetrical  in  respect 
to  those  characters  in  which  an  increase  was  shown  in  the 
first  generation.  This  criticism  has  its  basis  in  the  famil- 
iar fact  that  Mendelian  expectation  in  the  second  hybrid 
generation  where  there  is  complete  dominance  its  always 
an  expansion  of  the  form  (3  +  1)  to  a  power  represented 
by  the  number  of  factors.  Even  with  partial  dominance 
the  criticism  holds,  although  the  lack  of  symmetry  is  not 
so  marked. 

But  in  the  vast  amount  of  data  accumulated  upon  the 
inheritance  of  quantitative  characters  no  such  tendencies 
toward  asymmetrical  distribution  in  the  second  genera- 
tion are  evident.  In  the  majority  of  cases  recorded  where 
hybrid  vigor  is  shown  in  the  first  generation,  the  distri- 
bution of  the  individuals  fits  the  synometrical  curve,  com- 
monly known  as  the  Curve  of  Error,  remarkably  well. 

It  is  evident,  therefore,  that  the  objections  raised 
against  the  hypothesis  of  dominance  as  a  means  of  ac- 
counting for  heterosis,  as  outlined  by  Keeble  and  Pellew, 
are  valid.  But  both  these  objections  to  dominance  as  an 
interpretation  of  heterosis  were  made  before  the  facts  of 
linkage  were  known.  With  linkage  these  criticisms  based 
upon  Mendelian  expectancies  with  independent  factors  do 
not  hold. 

Abundant  evidence  is  fast  being  accumulated  to  show 


CAUSE  OF  HYBRID  VIGOR  173 

that  characters  are  inherited  in  groups.  The  different 
theories  accounting  for  this  linkage  of  factors  make  no 
essential  difference  in  the  use  to  which  these  facts  will  be 
put  here.  It  is  only  necessary  to  accept  as  an  established 
fact  that  characters  are  thus  inherited  and  that  it  is  these 
groups  of  factors  which  Mendelize.  The  chromosome 
view  of  heredity  will  be  used,  as  it  is  the  most  probable, 
the  most  useful,  and  permits  representation  in  the  sim- 
plest manner ;  but  adherence  to  this  view  is  not  necessary 
for  our  purpose. 

The  increasing  complexity  of  Mendelism  points  very 
strongly  to  the  probability  that  the  important  characters 
of  an  organism  are  determined,  or  at  least  affected,  by 
factors  represented  in  practically  all  of  the  chromosomes 
or  linkage  groups.  This  is  comprehensible  when  it  is  re- 
membered that  height  or  any  other  size  differentiation 
is  only  an  expression  of  an  organism *s  power  to  develop. 
Hereditary  factors  which  affect  any  part  of  the  organism 
may  indirectly  determine  the  maximum  of  any  size  char- 
acter. For  example,  in  plants  height  is  governed  by  root 
development  as  well  as  by  that  of  the  aerial  parts. 

The  widespread  occurrence  of  abnormalities  and  char- 
acters which  are  detrimental  to  an  organism  ^s  best  devel- 
opment are  well  known.  It  may  be  taken  for  granted, 
nevertheless,  that  no  one  individual  has  all  the  unfavor- 
able characters,  nor,  on  the  other  hand,  all  the  favorable 
characters  known  to  occur  in  the  species.  For  the  most 
part,  each  possesses  a  random  sample  of  the  good  and  the 
bad.  This  being  true,  it  is  only  necessary  to  assume  that 
in  general  the  favorable  characters  are  in  some  degree 
dominant  over  the  unfavorable,  and  the  normal  over  the 


174         INBREEDING  AND  OUTBREEDING 

abnormal  in  order  to  have  a  reasonable  explanation  of  the 
increased  development  of  hybrids  in  the  first  generation 
over  the  average  of  the  parents  or  subsequent  generations. 
In  the  first  hybrid  generation  the  maximum  number  of 
different  factors  can  be  accumulated  in  any  one  individ- 
ual ;  and  because  of  factor  linkage  it  is  extemely  difficult 
to  recombine  in  one  organism  in  later  generations  any 
greater  number  of  homozygous  characters  than  were 
present  in  the  parents,  provided  the  factors  are  distrib- 
uted at  random  in  all  of  the  chromosome  pairs.  This 
view  of  the  situation  makes  more  understandable 
why  the  effects  of  heterozygosis  result  in  an  increase 
in  development,  and  why  they  remain  throughout 
the  life  of  the  sporophyte,  even  though  innumerable 
asexual  generations. 

The  abstract  view  of  the  dominance  hypothesis  may 
be  somewhat  clearer  if  a  concrete  diagrammatic  illustra- 
tion is  made.  A  case  will  be  assumed,  in  which  two  homo- 
zygous individuals,  having  three  chromosome  pairs,  both 
attain  the  same  development  as  represented  by  any  meas- 
urable character.  This  development  will  be  considered  to 
amount  to  6  units,  2  of  which  are  contributed  by  each 
chromosome  pair.  One  of  these  individuals,  which  we  will 
call  **X,''  attains  its  development  through  the  operation 
of  factors  distributed  in  the  three  pairs  of  chromosomes, 
each  differing  from  the  others  in  its  contribution.  Any 
number  of  factors  can  be  chosen,  but,  for  the  sake  of  sim- 
plicity, only  three  in  each  chromosome  will  be  employed. 
These  are  numbered  1,  3,  5 ;  7,  9, 11 ;  and  13,  15, 17  in  the 
accompanying  diagram  (Fig.  36).  The  second  individual, 
**Y,*'  develops  to  an  equal  extent  in  the  character  meas- 


CAUSE  OF  HYBRID  VIGOR 


175 


ured.  It  attains  this  same  development,  however,  by  the 
operation  of  a  different  set  of  factors  distributed  in  the 
three  chromosomes  and  numbered  2,  4,  6 ;  8,  10,  12 ;  and 
14,  16,  18  in  the  diagram.    Both  individuals  are  homo- 


X:  6 


Y  :  6 


2 

2 

2 

2 

2 

2 

*A    BB    cc       a'/C   fi'e'Cri' 

1 

1 

7 

7 

13 

13 

^■^"^ 

3 

3 

9 

9 

15 

15 

2 

2 

8 

8 

14 

14 

S 

5 

11 

11 

17 

17 

4 
6 

4 
6 

10 
12 

10 
12 

16 
18 

16 
18 

Xx  Y:  12 


2 

2 

2 

2 

2 

2 

A   /    B   ef    c   d' 

1 

2 

7 

8 

13 

14 

3 

4 

9 

10 

15 

16 

s 

6 

11 

12 

17 

18 

Fia.  36. — To  show  how  factors  contributed  by  each  parent  may  enable  the  first  generation 
of  a  cross  to  obtain  a  greater  development  than  either  parent. 

zygous ;  i.e.,  the^  allelomorphic  pairs  are  composed  of  like 
elements.  It  is  also  assumed  that  all  these  nine  factors 
are  as  fully  effective  in  the  haploid  as  in  the  diploid  con- 
dition; in  other  words,  they  show  perfect  dominance  over 
their  absence.  It  will  be  seen  from  the  diagram  that  when 
these  individuals  are  crossed  together  the  progeny  de- 


176 


INBREEDING  AND  OUTBREEDING 


velop  to  twice  the  extent  of  either  parent,  because  there 
are  present  eighteen  different  factors  instead  of  nine. 

TABLE  VII 

Composition  of  a  Mendelian  Tri-Hybrid  in  Fj  Where  the  Development 
Which  Each  Individual  Attains  Depends  upon  the  Number  of 
Heterozygous  Chromosomes  Contained  and  Thereby  upon  the 
Total  Number  of  Different  Factors  Present. 


Number  of  individ- 

Contributions of 

Total 

uals  in  each 

Categories 

each  chromosome 

develop- 

category 

pair 

ment 

1 

A  A 

BB 

CC 

2+2+2 

6 

2 

A  A' 

B  B 

CC 

4+2+2 

8 

2 

A  A 

BB' 

CC 

2+4+2 

8 

2 

A  A 

BB 

CC 

2+2+4 

8 

4 

A  A' 

BB' 

CC 

4+4+2 

10 

4 

A  A 

BB' 

CC 

2+4+4 

10 

4 

A  A' 

BB 

CC 

4+2+4 

10 

8 

A  A' 

BB' 

CC 

4+4+4 

12 

1 

A  A 

BB 

CC 

2+2+2 

6 

2 

AA 

BB' 

CC 

2+4+2 

8 

2 

A  A' 

BB 

CC 

4+2+2 

8 

4 

A  A' 

BB' 

CC 

4+4+2 

10 

1 

A  A 

B'B' 

CC 

2+2+2 

6 

2- 

AA 

B'B' 

CC 

2+2+4 

8 

2 

A  A' 

B'B' 

CC 

4+2+2 

8 

4 

A  A' 

B'B' 

CC 

4+2+4 

10 

1 

A'A' 

BB 

CC 

2+2+2 

6 

2 

A'A' 

BB' 

CC 

2+4+2 

8 

2 

A'A' 

BB 

CC 

2+2+4 

8 

4 

A'A' 

BB' 

CC 

2+4+4 

10 

1 

A'A' 

B'B' 

CC 

2+2+2 

6 

2 

A'A' 

B'B' 

CC 

2+2+4 

8 

1 

A'A' 

BB 

CC 

2+2+2 

6 

2 

A'A' 

BB' 

CC 

2+4+2 

8 

1 

AA 

B'B' 

CC 

2+2+2 

6 

2 

A  A' 

B'B' 

CC 

4+2+2 

8 

1 

A'A' 

B'B' 

CC 

2+2+2 

6 

64  Total 

Distribution  of  the 

Fj 

individuals 

according 

to  the  development  attained 

Classes 

Frequency 

6 
8 

8 
24 

10 
24 

12 

8 

=  4 
=  64 

Number  of  classes 
Total  population 

Following  this  hypothetical  case  into  the  second  gen- 
eration by  selfing  or  by  interbreeding  the  individuals  of 


CAUSE  OF  HYBRID  VIGOR  177 

the  first  generation,  the  data  given  in  Table  VII  are  ob- 
tained. Summing  up  the  results  of  this  tabulation,  it  will 
be  found  that  eight  individuals  are  completely  homo- 
zygous and  reach  the  same  development  as  either  parent, 
six  units ;  eight  are  heterozygous  in  all  three  chromosome 
pairs  and  duplicate  the  twelve-unit  growth  of  the  first 
generation;  the  remaining  forty-eight  individuals  fall 
into  equal-sized  groups,  developing  to  eight  and  ten  units, 
respectively.  In  other  words,  the  distribution  is  sym- 
metrical, and  this  symmetry  remains,  however  many 
chromosomes  are  involved. 

It  should  also  be  noted  that  the  mean  development 
of  the  second  generation  is  nine  units,  which  is  an  excess 
of  just  half  of  the  excess  of  the  first  generation  over  the 
parent.  The  extra  growth  derived  by  crossing  the  two 
different  types  has  diminished  50  per  cent.  In  the  third 
generation,  from  a  representative  sample  of  the  second 
generation,  it  can  be  shown  that  this  excess  again  dimin- 
ishes 50  per  cent.,  so  that  the  effect  on  the  average  is  only 
25  per  cent,  as  great  in  this  generation  as  in  the  first,  and 
so  on,  in  subsequent  generations,  until  the  effect  dimin- 
ishes to  a  negligible  quantity  in  about  the  eighth  genera- 
tion. This  is  in  fair  agreement  with  the  actual  results 
obtained  by  inbreeding  maize,  as  it  ought  to  be,  because 
the  development  attained  by  each  individual  varies  di- 
rectly with  the  number  of  heterozygous  factors. 

In  the  preceding  illustration  of  the  way  heterosis 
may  be  brought  about,  perfect  dominance  was  assumed. 
Moreover,  breaks  in  linkage  with  the  formation  of 
new  linkage  groups  were  not  considered.  All  these 
things  enter  as  complicating  factors.  Perfect  domi- 
nance,  except  in  more  or  less   superficial   characters, 

12 


178         INBKEEDING  AND  OUTBEEEDING 

rarely  occurs,  and  even  when  it  does  occur,  it  may 
be  merely  an  appearance  rather  than  a  reality.  The  gen- 
eral consensus  of  opiaion  at  the  present  time  is  that  there 
is  no  such  thing  as  perfect  dominance,  that  the  hetero- 
zygote  merely  approaches  the  condition  of  one  or  the  other 
parent  more  or  less  closely.  When  two  different  poten- 
tialities are  contributed  by  the  parents,  there  results  an 
interaction  between  them  and  the  end  product  is  repre- 
sented in  the  organism.  Because  the  most  striking  effect 
may  resemble  the  character  of  one  parent  more  than  the 
other,  we  say  that  this  character  is  dominant.  In  reality, 
in  the  more  fundamental  characters,  the  hybrid  usually 
shows  a  resemblance  to  both  parents.  The  more  common 
illustrations  of  dominance,  such  as  fur  colors  and  flower 
colors,  probably  have  little  to  do  with  heterosis.  Other 
dominant  characters,  however,  have  a  fundamental  effect 
upon  development,  nearly  always  being  essential  to  great- 
est vigor.  Various  grades  of  albinism  are  common  in 
maize  and  in  many  other  plants.  Since  this  affects  the 
amount  of  chlorophyll,  the  presence  of  albinism  in  any 
form  seriously  retards  the  growth.  In  extreme  cases  the 
plants  are  totally  incapable  of  continuing  existence  be- 
yond the  stage  made  possible  by  food  stored  in  the  seed. 
In  animals,  albinism  does  not  have  the  physiological  sig- 
nificance that  it  has  in  plants,  but  even  here  it  is  some- 
times unfavorable  to  the  individuals  showing  it.  In  every 
case,  and  in  all  degrees,  true  albinism  is  recessive  to  the 
normal  condition.  In  maize,  the  heterozygous  green  plants 
camiot  be  distinguished  from  homozygous  green  plants. 
Many  other  unfavorable  characters  in  maize  are  also  re- 
cessive. Absence  of  brace  roots,  bifurcated  ears,  dwarf- 
ism, susceptibility  to  smut  all  behave  in  this  way. 


CAUSE  OF  HYBRID  VIGOR  179 

Certain  factors  have  even  been  recognized,  and  in  the 
case  of  Drosophila  ^^^'  ^^«  have  been  located  in  the  chromo- 
some mechanism,  which  are  so  injurious  that  they  cause 
the  death  of  the  individuals  possessing  them,  unless  pro- 
tected by  the  factors  being  in  combination  with  a  normal 
allelomorph.  A  well-known  case  of  this  kind  is  the  yellow 
mouse.  Lethal  factors,  in  order  to  be  recognized  easily, 
must  be  recessive  in  their  lethal  action  and  must  show  a 
visible  effect  on  the  soma  when  in  combination  with  their 
allelomorphs,  since  only  in  that  case  can  the  heterozygotes 
be  detected.  In  the  yellow  mouse  there  is  associated  with 
color  another  effect  which  causes  the  death  of  the  animals 
when  they  are  pure  for  that  factor.  This  has  been  demon- 
strated by  the  altered  ratios  obtained.  Yellow  mice  are 
mated  together ;  instead  of  getting  a  ratio  of  3  yellow  to  1 
non-yellow,  the  ratio  is  more  nearly  2:1;  that  is,  (1) :  2 : 1, 
in  which  the  pure  recessives  are  eliminated.^^  This  as- 
sumption is  further  corroborated  by  actually  finding  the 
missing  number  of  animals  in  stages  of  dissolution  in 
early  embryonic  life.  Of  the  more  than  one  hundred  and 
twenty-five  mutations  which  have  been  described  in  Dro- 
sophila by  far  the  greater  majority  of  them  are  recessive, 
and  nearly  all  of  them  are  less  favorable  to  the  develop- 
ment of  the  fly  than  the  wild-type  characters.  The  effect 
of  the  recessive  factors  even  seems  to  be  cumulative,  be- 
cause when  many  of  them  are  combined  together  the  flies 
are  extremely  difiScult  to  maintain.  Much  the  same  con- 
dition is  true  for  domesticated  animals  and  plants.  The 
majority  of  the  variations  which  have  occurred  are  reces- 
sive, and  are  seldom  beneficial  and  often  deleterious. 
/  One  may  be  led  to  inquire  why  it  is  that  most  of  the 
experimentally  observed  mutations  are  recessive  and  less 


180         INBREEDING  AND  OUTBREEDING 

favorable  to  the  best  development  of  the  organism.    We 
do  not  know,  but  we  may  hazard  a  guess.    The  repeated 
appearance  and  disappearance  of  certain  mutations  is 
merely  a  type  of  variability  which  has  probably  been  a 
constant  feature  of  the  organism  for  a  long  period  and 
has  been  subjected  to  natural  selection  in  the  same  way  as 
any  other  character.    In  other  words,  may  not  the  ten- 
dency to  produce  dominant  unfavorable  variations  have 
been  reduced  to  the  minimum  by  natural  selection?    Con- 
versely, a  tendency  to  produce  unfavorable  recessive  mu- 
tations has  been  tolerated  because  the  latter  are  pro- 
tected in  hybrid  combinations  by  their  dominant  favor- 
able allelomorphs.    Whether  this  be  true  or  not,  there  cer- 
tainly is  a  strong  tendency  for  dominant  unfavorable 
variations  to  be  eliminated,  because  they  are  constantly 
subjected  to  natural  selection;  while  dominant  favorable 
variations,  whenever  they  occur,  replace  former  charac- 
ters, and  become  part  of  the  stock  in  trade  of  the  organ- 
ism.   Recessive  mutations,  on  the  other  hand,  whether 
favorable  or  unfavorable,  cannot  compete  for  place  with 
natural  selection  as  the  judge  unless  the  proper  mating 
brings  them  into  the  homozygous  condition.    If  through 
continuous  cross  breeding  this  does  not  occur,  they  may 
be  carried  on  for  countless  generations — ^family  skeletons 
hidden  by  the  phenomenon  of  dominance. 

The  relation  of  these  reflections  to  heterosis  is  just 
this :  If  any  individual  is  deficient  and  handicapped  in  its 
hereditary  make-up,  there  is  a  good  chance  that  this  de- 
ficiency will  be  supplied  when  it  is  crossed  with  other 
individuals,  because  all  are  not  apt  to  be  wanting  in  the 
same  things.  What  one  lacks  is  supplied  by  the  other 
and  conversely.    In  other  words,  there  is  a  poolinsr  of 


y    CO 


w  P 

3  EL 
3  o 
2  CO 

»^ 


?3    CO 

tB    O 
<t>    CO 


O  3 
3  P 

If 

-    y 

o   »' 


4 


CAUSE  OF  HYBRID  VIGOR  181 

hereditary  resources,  so  that  the  combined  effect  is  better 
than  either  could  produce  alone. 

This  complementary  action  can  be  illustrated  by  as- 
suming three  linked  factors,  all  of  which  are  essential  for 
best  development.  In  one  organism  there  is  AbC;  in  an- 
other there  is  oBc,  Dominance,  either  partial  or  complete, 
is  characteristic  of  each.  Now  in  the  former  interpretation 
of  heterosis,  where  a  physiological  stimulation  was  as- 
sumed, the  heterozygous  combination,  Aa,  for  example, 
evolved  developmental  energy  and  differed  in  that  respect 
from  either  AA  or  aa.  Moreover,  this  stimulation  was 
considered  to  be  of  a  general  nature,  affecting  the  organ- 
ism as  a  whole,  and  was  thus  differentiated  from  the  spe- 
cific effect  which  each  had  as  hereditary  factors.  With 
linkage,  one  may  consider  heterosis  to  be  due  to  the  action 
of  heredity  alone :  the  hybrid  union  Aa  is  not  superior  to 
either  of  the  homozygous  combinations,  AA  or  aa,  but  is 
more  or  less  intermediate.  This  view  has  a  very  great 
theoretical  and  practical  importance,  because  one  may  ex- 
pect to  obtain  homozygous  instead  of  heterozygous  com- 
binations of  the  factors  which  bring  about  increased  vigor 
in  crosses  and  thus  obtain  individuals  which  will  have  a 
vigor  equal  or  even  superior  to  the  first  cross  and  which 
will  not  be  affected  by  future  inbreeding. 

Such  a  happy,  result  was  not  possible  on  the  stimula- 
tion hypothesis.  This  hypothesis  was  invented  to  account 
for  the  frequency  of  heterosis,  the  loss  of  vigor  due  to 
inbreeding,  and  the  extreme  rarity  of  homozygous  com- 
binations approaching  those  of  the  first  hybrid  generation 
in  vigor.  With  a  knowledge  of  independent  Mendel i  an 
heredity  only  it  was  necessary.  But  if  in  our  illustration 
the  individual  AbC-aBc  is  vigorous  because  of  heredity 


^ 


182         INBEEEDING  AND  OUTBREEDING 

alone,  and  if  it  usually  segregates  germ  cells  of  the  types 
AbC  and  aBc,  making  the  vigor  thus  obtained  a  practical 
corollary  of  heterozygosity,  there  is  still  the  chance,  no 
matter  how  closely  linked  these  factors  may  be,  of  breaks 
occurring  which  will  bring  about  the  production  of  a  gam- 
ete ABC.  This  gamete,  if  it  meets  another  of  the  same 
type,  will  result  in  a  homozygous  individual,  and  if  dom- 
inance is  but  partial,  this  individual,  through  the  very 
fact  of  its  homozygous  condition,  will  be  even  more  vig- 
orous than  those  of  the  first  hybrid  generation. 

Practically  the  difficulties  in  the  way  of  obtaining  such 
pure  combinations  may  be  very  great  or  even  insurmount- 
able, but  the  hypothesis  holds  out  the  hope  of  thus  ob- 
taining types  of  great  economic  value.  The  rearrange- 
ment of  factors  in  all  possible  recombinations  is  not  pre- 
vented by  linkage  as  long  as  there  are  breaks  in  the 
linkage.  But  since  these  breaks  occur  with  varied  fre- 
quency between  different  factor  loci,  and  in  some  cases  are 
very  rare,  the  problem  is  exceedingly  complicated;  and 
when  many  linked  factors  are  involved  the  chance  of  ob- 
taining an  individual  which  is  completely  homozygous 
in  all  factors  in  the  first  segregating  generation  is  so  ex- 
tremely small  that  for  all  practical  purposes  it  may  be 
disregarded.  In  later  generations  the  chance  may  become 
somewhat  greater  because  of  the  formation  of  new  linked 
groups.  If  these  are  combinations  which  are  favorable  to 
development,  natural  selection  will  increase  individuals 
possessing  them  at  the  expense  of  those  having  less  favor- 
able combinations.  In  time,  there  is  the  possibility,  how- 
ever remote,  of  all  the  more  favorable  factors  being 
brought  together  in  a  homozygous  combination  which, 
therefore,  will  not  be  reduced  by  inbreeding. 


CAUSE  OF  HYBRID  VIGOR  183 

Our  hypothetical  iUustration  of  characters  contrib- 
uted by  both  parents  may  be  supported  by  actual  results 
from  a  cross  between  inbred  strains  of  maize.  As  men- 
tioned before,  maize  strains  have  been  obtained  which  lack 
brace  roots  and  are  unable  to  stand  upright  when  the 
plants  become  heavy.  These  strains,  however,  have  the 
habit  of  branching  freely  from  the  base  of  the  plant,  thus 
producing  several  main  stalks  from  each  seed.  When  this 
strain  is  crossed  with  one  which  possesses  well-developed 
roots,  but  which  does  not  branch  at  the  base  of  the  plant, 
the  result  is  an  extremely  vigorous  progeny  which  pro- 
duces several  stalks  from  each  seed  and  which  shows  no 
deficiency  in  root  development.  The  hybrid  plants  are 
so  large  and  so  exceedingly  vigorous  that  other  factors 
must  have  been  involved,  but  these  two  characters  can 
easily  be  seen  to  have  contributed  something  to  the 
luxuriant  development. 

An  even  more  striking  illustration  was  obtained  by 
Emerson.  A  dwarf  race  of  maize  which  was  almost  com- 
pletely sterile  was  crossed  with  a  tall  plant  which  was 
so  deficient  in  chlorophyll  production  that  it  was  un- 
able to  produce  seed,  although  it  had  some  functional 
pollen.  The  hybrid  plants  were  tall,  dark  green  and 
produced  well-developed  ears.  Here  normal  stature  was 
contributed  by  one  parent  and  proper  chlorophyll  devel- 
opment by  the  other,  the  progeny  was  thereby  enabled  to 
develop  well  and  to  become  highly  productive. 

In  both  these  crosses  the  characters  involved  are 
largely  of  a  superficial  nature,  although  most  of  them  are 
necessary  for  full  development.  They  are  characters 
which  are  easily  seen  and  serve  as  an  indication  of  what 
must  have  occurred  in  the  case  of  other  factors  more  in- 


184         INBEEEDING  AND  OUTBREEDING 

temal  in  their  effect  and  of  more  fundamental  importance. 
These  more  fundamental  factors  are  those  concerned  di- 
rectly with  metabolism  and  cell  division.  As  to  their  na- 
ture and  the  way  in  which  they  are  inherited,  we,  as  yet, 
know  little,  but  there  is  reason  for  supposing  they  are 
Mendelian  in  mode  of  inheritance  and  operate  in  a  way 
to  enable  the  hybrid  progeny  to  attain  a  greater  develop- 
ment than  either  parent. 

For  a  definite  detailed  case  showing  exactly  how  dom- 
inance and  linkage  thus  work  together  we  must  look  to  the 
work  with  Drosophila  melanogaster,  as  this  is  the  only 
material  which  at  the  present  time  has  been  sufficiently 
well  analyzed  for  our  purpose.  Bridges  and  Sturtevant 
have  discovered,  isolated,  and  determined  the  linkage  re- 
lations of  nearly  one  hundred  factors  distributed  through- 
out the  four  chromosomes  of  this  little  fly,  in  the  great 
majority  of  which  the  recessive  condition  is  unfavorable. 
Through  this  indefatigable  work  there  is  an  enormous 
amount  of  data  from  which  to  choose ;  but  in  order  to  make 
our  illustration  comparatively  easy  to  follow,  let  us  con- 
sider only  four  characters  which  are  linked  together  in  the 
second  chromosome.  The  factors  which  have  the  princi- 
pal effect  on  these  characters  may  be  given  the  name 
of  the  character.  They  are  long  legs  (D)  dominant  to 
**dachs"  legs  (d)^  gray  body  (B)  dominant  to  black  body 
(&),  red  eye  (P)  dominant  to  purple  eye  (p),  and  normal 
wings  (V)  dominant  to  vestigial  wings  (v).  In  gamete 
formation  in  the  female  there  are  breaks  with  a  frequency 
of  10  per  cent,  in  the  linkage  between  d  and  h,  of  6  per 
cent,  between  h  and  p,  and  of  13  per  cent,  between  p  and  v, 
disregarding  some  disturbing  conditions  which  need  not 
concern  us  here.   In  the  male  there  are  no  linkage  breaks. 


CAUSE  OF  HYBRID  VIGOR  185 

Now  if  a  female  fly  with  dachs  legs,  gray  body,  purple 
eyes  and  normal  wings  (dBpV),  be  crossed  with  a  male 
having  long  legs,  black  body,  red  eyes  and  vestigial  wings 
{DbPv)y  the  resulting  progeny  will  have  the  usual  wild- 
type  characters,  long  legs,  gray  body,  red  eyes  and  normal 
wings,  and  will  be  considerably  more  vigorous  than  either 
parent.  If  these  factors  segregated  independently,  one 
would  expect  to  find  one  gamete  out  of  every  sixteen  to  be 
of  the  constitution  DBPV,  and  would  obtain  one  F2  indi- 
vidual homozygous  for  this  combination  of  the  four  domi- 
nants out  of  every  two  hundred  and  fifty-six.  As  a  mat- 
ter of  fact,  owing  to  the  linkage  relations  found,  only  one 
gamete  of  this  kind  is  produced  in  two  thousand  and  then 
only  in  the  female.  It  is,  therefore,  impossible  to  obtain 
the  type  sought  in  the  F2  generation.  But  males  of  the 
all-dominant  type  will  appear  in  Fj,  and  the  pure  strain 
may  be  established  in  F^.  The  word  '^may'^  is  used  as  a 
sort  of  forlorn  hope,  however.  There  is  a  possibility  of 
establishing  the  homozygous  dominant  strain  in  F^,  but 
when  one  realizes  that  in  F2  only  one  such  male  and  one 
heterozygous  female  similar  in  appearance  to  hundreds  of 
her  sisters  will  be  produced  in  every  four  thousand  pro- 
geny, the  difficulties  in  the  situation  are  emphasized. 

The  frequency  of  the  linkage  breaks  is  large  and  the 
number  of  factors  smaU  in  this  illustration.  When  it  is 
remembered  that  in  other  organisms  there  are  ten,  twenty, 
or  even  forty  chromosome  pairs  to  be  considered,  with 
possibly  dozens  of  factor  differences,  instead  of  four  in 
each  chromosome,  some  idea  may  be  obtained  of  the  real 
difficulties  involved  in  producing  individuals  of  maximum 
vigor  unaffected  by  inbreeding.  Practically  speaking,  it 
is  impossible  unless  dealing  with  a  small  number  of 


186         INBEEEDING  AND  OUTBREEDING 

loosely  linked  factors,  except  when  long  periods  of  time 
are  available  and  when  natural  elimiriation  of  undesir- 
ables is  high. 

In  tracing  the  evolution  of  ideas  concerning  the  effects 
of  inbreeding  and  outbreeding  we  must  give  great  credit 
to  Darwin  for  calling  attention  to  the  importance  of  the 
phenomena  in  relation  to  evolution  and  for  being  the  first 
to  see  that  hereditary  differences,  rather  than  the  mere  act 
of  crossing,  was  the  real  point  involved ;  but  with  all  due 
credit  to  Darwin,  it  was  not  until  Mendelism  became 
known,  appreciated  and  applied  that  the  first  real  attack 
upon  the  problem  was  made  possible.  When  linked  with 
Mendelian  phenomena  it  was  clearly  recognized  for  the 
first  time  that  one  and  the  same  principle  was  involved  in 
the  effects  of  inbreeding  and  the  directly  opposite  effects 
of  outbreeding.  Inbreeding  was  not  a  process  of  continual 
degeneration.  Injurious  effects,  if  present,  were  due  to 
the  segregation  of  characters.  In  addition  to  this  segre- 
gation of  characters  the  fact  was  established  that  an  in- 
creased growth  accompanied  the  heterozygous  condition. 
All  the  essential  facts  were  accounted  for.  A  decade  later 
the  great  extension  of  knowledge  in  the  field  of  heredity 
has  made  possible  a  still  closer  linking  of  the  facts  of  in- 
breeding and  outbreeding  with  Mendelism.  The  hypothe- 
sis of  the  complementary  action  of  dominant  factors  is 
the  logical  outgrowth  of  former  views  and  makes  the  in- 
creased growth  of  hybrids  somewhat  more  understand- 
able. The  fact  of  a  stimulation  accompanying  hetero- 
zygosity is  supplemented  by  a  reason  why  such  an  effect 
is  obtained.  The  former  view  of  a  physiological  stimula- 
tion and  the  more  recent  conception  of  the  combined  ac- 


CAUSE  OF  HYBRID  VIGOR  187 

tion  of  dominant  factors  are  not  then  two  unrelated  hy- 
potheses to  be  held  up  for  the  choosing  of  the  one  from 
the  other.  The  outstanding  feature  of  the  latter  view  is 
that  there  is  no  longer  any  question  as  to  whether  or  not  in- 
breeding as  a  process  in  itself  is  injurious.  Homozygosity, 
when  obtained  with  the  combination  of  all  the  most  favor- 
able characters,  is  the  most  effective  condition  for  the 
purpose  of  growth  and  reproduction. 


I 


CHAPTER  IX 

STEEILITY  AND  ITS  RELATION  TO  INBREEDING 

AND  CROSS-BREEDING 

Probably  the  most  noticeable  effect  of  inbreeding  in 
both  animals  and  plants  is  a  reduction  in  fertility  in  the 
earlier  inbred  generations.  The  experiments  of  Ritzema- 
Bos^^*  with  rats,  of  Weismann^^  with  mice  and  of 
Wright  2^^  with  guiaea-pigs  are  all  thus  characterized. 

Miss  King/^^  on  the  other  hand,  has  iabred  albino 
rats  for  twenty-five  successive  generations  by  brother  and 
sister  matiQg  without  any  appreciable  reduction  in  fer- 
tility. Similarly,  Castle  ^^  and  his  students  maintained 
the  fertility  of  Drosophila  for  fifty-nine  generations  of 
brother  and  sister  mating  by  breediag  from  the  most  fer- 
tile flies.  Various  lines  were  isolated,  nevertheless,  which 
differed  in  the  number  of  offspring  produced,  and  in  the 
first  part  of  the  experiment  many  individuals  appeared 
which  were  absolutely  sterile.  The  production  of  such 
non-fertile  flies  became  less  in  the  latter  part  of  the  ex- 
periment, and  the  average  fertility  of  the  remaining  stock 
was  improved  by  this  elimination. 

In  maize  the  results  of  inbreeding  are  generally  quite 
serious  as  regards  fertility.  In  the  first  place  the  consis- 
tent reduction  in  size  and  constitutional  vigor  of  the 
plants  necessitates  a  much  smaller  production  of  pollen 
and  ovules.  The  tassels  are  reduced  in  size  and  have 
fewer  branches.  The  ears  are  smaller  and  shorter  and 
oftentimes  imperfectly  covered  with  seeds  even  when 
abundant  pollen  is  available.    In  some  cases  the  leaves 

188 


STERILITY  189 

enclosing  the  tassels  do  not  unfold  properly  and  the  tas- 
sel does  not  develop  as  it  should.  This  is  a  secondary 
effect,  but,  nevertheless,  is  one  factor  in  reducing  fertil- 
ity. The  anthers  are  frequently  much  shruuken,  some- 
times shedding  no  pollen  at  all  and  even  under  the  best 
conditions  producing  a  very  meagre  supply.  The  amount 
of  pollen  produced  is  more  affected  by  weather  conditions 
in  such  inbred  strains  than  in  more  vigorous  plants.  At 
the  same  time  inbred  strains  of  maize  have  been  obtained 
which  show  no  degeneration  in  the  staminate  parts.  Their 
anthers  are  full  and  produce  abundant  pollen.  Such 
strains  thus  far  have  been  few  in  number.  They  are  cor- 
related with  poor  development  of  the  pistillate  parts. 
Those  strains  which  have  the  best  developed  ears  as  a 

.,  rule  have  very  much  reduced  tassels  with  a  large  amount 
of  pollen  abortive.  Some  strains  have  been  obtained  which 
are  about  equally  well  developed  in  both  staminate  and 
pistillate  functions,  and  these  range  all  the  way  from 
plants  which  are  fairly  productive  for  inbred  strains  down 
to  types  which  barely  produce  enough  seed  to  survive,  and 
since  many  cases  of  failure  to  produce  seed  are  met  there 
can  hardly  be  a  doubt  that  in  some  of  them  a  complete 

I  abortion  of  one  or  both  functions  has  taken  place.  As  in 
the  many  other  effects  of  inbreeding  different  results  are 
produced  in  different  lines,  showing  clearly  that  segrega- 
tion of  certain  factors  influencmg  fertility  has  taken  place. 
On  the  whole,  there  is  in  this  species  a  tendency  for  in- 
breeding to  result  in  a  change  from  a  monoecious  condi- 
tion to  a  functionally  dioecious  condition. 

Sterility  in  the  form  of  structural  degeneration  whon 
it  occurs  gradually  increases  upon  inbreeding  until  homo- 
zygosity is  attained,  but  for  the  most  part  it  does  not  show 


190         INBREEDING  AND  OUTBREEDING 

any  clear-cut  segregation.  Yet  reduction  in  fertility  is 
noticeable  only  so  long  as  there  is  a  change  in  other  char- 
acters, constancy  in  visible  characters  being  accompanied 
by  a  constancy  in  the  matter  of  fertility.  In  other  words, 
there  is  no  more  an  accumulation  of  sterility  on  con- 
tinued inbreeding  than  there  is  an  accumulation  of  any 
other  effect.  Any  reduction  in  fertility  ceases  when 
homozygosity  is  reached,  but  the  end  result  may  be 
decidedly  different  in  various  lines  coming  originally  from 
the  same  stock. 

Many  other  instances  of  an  effect  of  inbreeding  upon 
fertility  might  be  given,  particularly  the  appearance  of 
abnormalities  in  the  genital  organs,  both  external  and  in- 
ternal. But  what  we  desire  is  to  show  their  meaning 
rather  than  to  catalogue  them,  and  for  this  purpose  no 
data  have  been  gathered  as  valuable  as  those  upon  the 
much  cited  maize.  Examination  of  all  the  isolated  facts 
brought  to  light  in  both  animals  and  plants  shows  such  a 
similar  trend  that  there  is  no  reason  to  believe  we  are  not 
dealing  with  manifestations  of  one  and  the  same  law,  yet 
only  in  this  species  do  we  have  a  critical  test  of  the  hypoth- 
eses involved.  And  here  it  can  be  stated  unequivocally 
and  without  reservation  that  the  effect  of  inbreeding  on 
fertility  is  exactly  the  same  as  its  effect  upon  other  char- 
acters. Recessive  combinations  deleterious  to  the  func- 
tion of  reproduction  are  brought  to  light.  But  this  is  not 
the  only  conclusion  to  be  drawn.  The  frequency  with 
which  depression  of  fertility  occurs  during  inbreeding,  the 
slowness  with  which  it  is  brought  to  an  end,  the  variety 
of  differences  which  is  brought  out,  all  show  how  complex 
one  must  conclude  the  function  of  reproduction  to  be,  and 
how  many  variations  affecting  it  must  be  constantly  occur- 


STEEILITY  191 

ring.  In  other  words,  there  is  here  a  concrete  illustration 
of  the  primaiy  importance  of  reproduction  in  all  evolu- 
tion. Since  provision  for  succession  exceeds  all  other 
matters  in  import  to  the  species,  new  variations  are  con- 
stantly taking  place,  new  processes  are  continuously  being 
tried  out.  The  result  is  to  have  reproduction  tied  up  with 
more  complications  than  any  other  physiological  process, 
to  have  in  naturally  cross-bred  species  more  heterozygous 
factors  than  in  any  other  character  complex.  Repro- 
duction, therefore,  as  we  have  seen,  is  affected  more 
often  and  more  frequently  than  anything  else  when 
inbreeding  occurs. 

When  inbred  strains  showing  reduced  fertility  are 
crossed,  on  the  other  hand,  there  is  almost  always  a  return 
to  the  original  productiveness  along  with  the  return  to  the 
original  size  and  vigor.  In  fact,  just  as  fertility  is 
affected  adversely  by  inbreeding  more  than  any  other 
character,  so  is  it  increased  more  in  proportion  by  cross- 
ing. But  such  increases  in  productiveness  are  the  rule 
only  up  to  a  certain  point  of  germinal  difference  between 
the  individuals  taking  part  in  the  cross.  As  dissimilarity 
in  the  uniting  germ  plasms  becomes  greater,  sterility 
again  manifests  itself.  This  time,  however,  the  sterility 
shown  is  of  a  different  nature.  No  structural  abnormali- 
ties appear.  There  are  no  variations  such  as  are  found  in 
the  numerous  strains  differentiated  by  inbreeding.  It  is 
simply  a  matter  of  non-production  of  functional  gametes. 

Based  upon  these  germinal  differences  crosses  be- 
tween species  may  be  classified  arbitrarily  as  follows : 

(1)  The  hybrid  may  have  the  same  or  greater  vigor 
and  fertility  than  the  parents.  Nicotiana  alata  and  N. 
Langsdorffii,  for  example,  are  distinct  species  having  dif- 


192         INBEEEDING  AND  OUTBREEDING 

ferences  in  many  oliaraoters,  yet  their  hybrids  give  no 
indication  of  any  lessened  fertility. 

(2)  The  hybrid  may  have  the  same  or  greater  vigor 
than  the  parents  and  at  the  same  time  show  reduced  fer- 
tility or  even  total  sterility.  This  is  a  common  result  with 
many  species  hybrids.  The  increase  in  growth  is  often- 
times extreme.  The  cross  between  the  garden  radish, 
Raphanus  sativus,  and  the  cabbage,  Brassica  oleracea, 
two  species  belonging  to  different  genera,  gives  plants  of 
rampant  growth  which  are  very  nearly,  if  not  completely, 
sterile,  as  shown  by  Sageret  ^^^  nearly  a  century  ago  and 
by  Gravatt  ^^  in  recent  times  (Fig.  38).  In  most  of  these 
cases  no  seed  can  be  produced  by  self-fertilization,  but 
back  crossing  with  one  or  the  other  parents  is  sometimes 
successful.  Animal  hybrids  frequently  show  sterility  in 
the  males  and  partial  or  complete  fertility  in  the  females. 
This:  is  the  condition  in  Cavia  species  hybrids  (Detlef- 
sen^*^)  and  in  crosses  made  between  the  buffalo,  Bison 
(MYiericanus;  the  yak,  Bihos  grumens;  the  gayal,  Bibos 
frontalis;  the  gaur,  Bibos  gaurus,  and  the  domestic  cow. 
Bos  taurus, 

(3)  The  species  hybrid  may  exhibit  a  reduced  size  and 
a  decline  in  vigor  combined  with  complete  sterility.  The 
phenomenon  shows  in  various  degrees.  For  example, 
East  and  Hayes  ^^  made  several  Nicotiana  crosses  in 
which  the  seed  would  not  germinate,  although  both  em- 
bryo and  endosperm  tissue  was  formed.  Crosses  between 
Nicoticma  tahacum  and  N.  paniculata,  and  between  N. 
rustica  and  N.  alata  resulted  in  seed  which  germinated, 
but  the  plants  were  weak  and  died  before  flowering,  appar- 
ently because  of  inability  to  utilize  the  starch  formed. 


Fig.  38. — Sterile  hybrid  between  radish  and  cabbage,  showing  the  rampant  growth,  aoconi 
panied  by  steriUty,  sometimes  obtained  in  wide  crosses.    (Gravatt,  in  .Jour.  Iliredity.) 


STEEILITY  193 

In  other  crosses  the  plants  matured,  but  they  developed 
very  slowly  and  in  the  end  were  smaller  than  either 
of  the  parents. 

In  general,  therefore,  it  can  be  said  that  differences  in 
uniting  germ  plasms,  when  not  too  great,  may  bring  about 
both  more  efficient  development  and  increased  fertility. 
Beyond  that  critical  point  of  difference  both  fertility  and 
vigor  may  be  decreased,  but  fertility  is  usually  the  hrst 
to  suffer— even  complete  sterility  often  being  coupled 
with  rampant  growth.  Nature  thus  steps  in  before  a 
germinal  heterogeneity  which  will  endanger  the  health  of 
the  hybrid  organism  has  been  reached,  and  prevents  mul- 
tiplication entirely.  This  is  an  important  physiological 
provision,  since  when  great  germinal  differences  exist 
there  is  reduced  growth  as  well  as  sterihty.  Groups  are 
thus  set  apart  which  may  evolve  within  themselves  by  put- 
ting to  good  use  heterosis  and  Mendelian  recombination. 
What  apparently  happens  is  this :  As  germinal  differences 
increase  a  point  is  reached  at  which  the  precise  and  com- 
plex machinery  governing  gametogenesis  cannot  do  its 
work  in  the  normal  manner  and  sterility  results,  although 
under  the  same  conditions  developmental  cell  division 
goes  on  as  usual.  Beyond  this  degree  of  difference  in  the 
uniting  germ  plasms,  even  somatic  cell  division  is  affected. 

This  sterility  accompanying  wide  crosses  is  an  almost 
untouched  problem.  We  can  throw  no  light  upon  it  except 
the  suggestions  noted  in  the  last  few  sentences.  For  this 
reason  one  may  inquire  why  it  is  mentioned  in  this  con- 
nection at  all.  In  spite  of  our  comparative  lack  of  knowl- 
edge as  to  just  what  occurs  in  the  cell  divisions  of  vdde 
crosses,  however,  there  is  an  excuse  for  meddling.  The 
peculiar  resemblance  of  the  effect  of  inbreeding  to  the 

13 


194         INBEEEDING  AND  OUTBEEEDING 

effect  of  crossing  various  distinct  species,  has  led  many- 
writers  to  identify  the  phenomena.  Further,  several 
critics  have  maintained  that  a  theory  which  purports  to 
interpret  sterility  in  the  one  case,  should  interpret  it  in 
the  other.  Now  this  is  a  point  of  view  which  is  obviously 
incorrect  even  with  our  present  meagre  knowledge  of  the 
facts.  The  sterility  often  accompanying  inbreeding  is 
not  the  same  thing  as  the  sterility  resulting  from  hybrid- 
ization. The  resemblance  is  superficial  in  the  extreme. 
In  the  one  case  there  is  the  differentiation  of  distinct 
strains  differing  anatomically  and  physiologically  in  their 
ability  to  perform  the  act  of  reproduction.  It  is  a  phe- 
nomenon of  Mendelian  heredity  which  stands  out  in  the 
clear-cut  manner  it  does,  because  the  progenitors  of  the 
individuals  thus  characterized  have  gone  through  with  the 
mechanical  process  which  segregates  factors,  in  the  pre- 
cise manner  necessary  to  accomplish  the  purpose.  In  the 
other  case,  the  individuals  are  sterile  because  they  cannot 
go  through  this  same  process  in  the  exact  and  proper  way 
required,  on  account  of  the  incompatibility  of  the  unit- 
ing cells. 


CHAPTER  X 

THE  EOLE  OF  INBEEEDING  AND  OUTBREEDING 

IN  EVOLUTION 

In  our  brief  consideration  of  the  more  important 
changes  which  have  occurred  in  the  reproductive  mechan- 
isms of  animals  and  plants,  several  features  stand  out 
impressively.  Both  animals  and  plants  have  followed 
modes  of  reproduction  that  are  identical  in  what  are 
deemed  to  be  the  essential  features,  something  which  can 
be  said  of  no  other  life  process.  It  is  not  enough  simply 
to  say  that  sexual  reproduction  has  become  the  dominant 
mode  of  propagation  among  organisms.  One  must  go 
further.  Cross-fertilization,  either  continuous  or  occa- 
sional, is  the  really  successful  method  of  multiplication 
everywhere.  Such  a  parallel  evolution  in  the  two  king- 
doms is  valid  evidence  of  real  worth  in  the  process :  a 
consideration  of  the  evidence  on  inbreeding  and  cross- 
breeding permits  us  to  state  this  value  in  concrete  terms. 

The  establishment  of  methods  of  reproduction  which 
maintain  variation  and  inheritance  mechanisms  on  a  high 
plane  of  efficiency  is  naturally  a  fundamental  requirement 
in  evolution.  Since,  however,  we  have  seen  that  there  is 
no  reason  for  believing  sexual  reproduction  to  be  better 
adapted  to  assure  a  numerous  progeny  than  asexual  re- 
production, it  either  must  be  a  more  perfect  means  of 
hereditary  transmission,  or  it  must  offer  selective 
agencies  a  greater  variety  of  raw  material. 

Fortunately  we  are  able  to  eliminate  the  first  alter- 
native.   There  is  definite  evidence  that  sexual  reprodnc- 

195 


196         INBEEEDING  ^ND  OUTBREEDING 

tion  does  not  differ  from  asexual  reproduction  in 
what  may  be  called  the  heredity  coefficient.  It  holds 
out  no  advantage  as  an  actual  means  for  the  transmis- 
sion of  characters. 

The  majority  of  zoological  data  on  this  subject  has 
little  value  on  account  of  the  experimental  difficulties  in- 
herent in  the  material,  although  zoologists  have  published 
more  on  the  matter  than  the  botanists.  Plants  furnish  the 
best  material  because  of  the  ease  in  handling  large  num- 
bers of  both  cuttings  and  seedlings  side  by  side,  and  be- 
cause of  the  opportunity  to  utilize  hermaphroditic  species. 
Even  with  the  best  plant  material  several  undesired  vari- 
ables are  present,  and  experiments  with  them,  therefore, 
are  not  without  their  disappointments ;  but  no  one  who 
has  had  a  long  and  intimate  experience  in  handling  pedi- 
gree cultures  of  plants  can  have  any  doubts  concerning 
the  correctness  of  our  conclusion.  Practically  the  inquiry 
must  take  the  form  of  a  comparison  between  the  varia- 
bility of  a  homozygous  race  when  propagated  by  seeds 
and  when  propagated  by  some  asexual  method.  The  first 
difficulty  is  that  of  obtaining  a  homozygous  race  and  thus 
eliminating  Mendelian  recombination.  The  traditionally 
greater  variability  of  seed-propagated  strains  is  due 
wholly  to  this  difficulty,  we  believe.  It  may  be  impossible 
to  obtain  a  race  homozygous  in  all  factors.  There  may  be 
a  physiolo.2:ical  limit  to  homozygosis  even  in  hermaphro- 
ditic plants.  The  best  one  can  do  is  to  use  a  species 
which  is  naturally  self-fertilized,  relying  on  continued 
self-fertilization  for  the  elimination  of  all  the  hetero- 
zyarous  characters  possible.  We  have  examined  many 
populations  of  this  character  in  the  genus  Nicotiana  and 
have  been  astounded  at  the  extremely  narrow  variability 


\/'l- 


■^  ^  ^«. 


Fig.    39.  —  Tassels  and  ears  of  an  almost  sterile  strain   obtained   by    inbrcodinfi   maize. 

(From  Emerson.) 


EVOLUTION  197 

they  exhibit.  Even  though  one  cannot  grow  each  member 
of  such  a  population  under  identical  conditions  as  to  nutri- 
tion, the  plants  impress  one  as  if  each  had  been  cut  out 
with  the  same  die.  Qualitative  characters  such  as  color 
show  no  greater  variation,  as  far  as  human  vision  may 
determine,  than  descendants  of  the  same  mother  plant 
propagated  by  cuttings.  Further,  in  certain  characters 
affected  but  slightly  by  external  conditions,  such  as  flower 
size,  the  sexually  produced  population  not  only  shows  no 
greater  variability  than  the  asexually  produced  popula- 
tion, but  it  shows  no  more  than  is  displayed  by  a  single 
plant.  Yet  one  must  remember  that  in  such  a  test  the 
seeds  necessarily  contain  but  a  small  quantity  of  nutrients 
and  for  this  reason  the  individual  plants  are  produced 
under  somewhat  more  varied  conditions  than  those  result- 
ing from  cuttings,  hence  it  would  not  have  been  unreason- 
able to  have  predicted  a  slightly  greater  variability  for 
the  sexually  produced  population,  even  though  the  coefi5- 
cient  of  heredity  of  both  were  the  same.  Similar,  though 
less  systematic,  observations  have  been  made  on  wheat — 
an  autogamous  plant  almost  as  satisfactory  for  such  a 
test  as  Nicotiana — with  practically  identical  results. 

One  is  justified,  then,  in  claiming  there  is  experi- 
mental evidence  to  show  that  sexual  reproduction  in 
itself  is  no  more  than  an  exact  equivalent  of  asexual  re- 
production in  the  matter  of  an  heredity  coefficient.  But 
is  this  also  true  for  germinal  variation?  We  believe  it  is. 
Variations  similar  in  size  and  kind  arise  both  in  asexual 
and  in  sexual  reproduction,  but  it  cannot  be  maintained 
they  occur  more  frequently  in  the  latter.  There  are  insects 
in  Oligocene  amber  apparently  identical  with  those  of 
to-day,  proving  constancy  of  type  to  be  possible  under 


198         INBEEEDING  AND  OUTBEEEDING 

sexual  reproduction  through  millions  of  years ;  there  are 
asexually  reproduced  species  of  plants  just  as  constant 
and  probably  stiLL  more  ancient.  At  the  same  time,  ger- 
minal variations  occur  to-day  under  sexual  reproduction 
in  somewhat  noteworthy  numbers,  as  Morgan's  work  on 
Drosophila  shows.  There  has  been  no  trustworthy  esti- 
mation of  their  frequency  within  even  a  single  species,  but 
it  cannot  be  said  they  occur  in  less  numbers  than  where 
asexual  reproduction  rules,  even  among  organisms  of  a 
relatively  high  specialization.  If  there  are  those  who 
doubt  this  statement,  let  them  refer  to  the  huge  list  of  bud- 
variations  in  the  higher  plants  compiled  by  Cramer.^^ 
He  will  be  able  to  identify  there,  type  by  type,  class 
by  class,  practically  all  of  the  variations  he  is  able  to 
discover  in  the  same  species  in  the  literature  on  sem- 
inal reproduction. 

It  is  rather  odd  that  this  should  be  the  case,  for  what  is 
being  discussed  here  is  not  really  the  frequency  with 
which  variations  occur,  but  rather  the  frequency  with 
which  they  are  detected.  And  theoretically,  the  ease  of 
detecting  variations  ought  not  to  be  tlie  same  under  the 
different  modes  of  reproduction.  If  it  be  granted  that 
changes  in  the  constitution  of  the  chromosomes  are  direct 
causes  of  variations,  and  that  such  changes  in  constitu- 
tion are  equally  probable  in  all  chromosomes,  it  follows 
that  partheno genetic  individuals  having  the  haploid  num- 
ber of  chromosomes  should  show  a  larger  proportion  of 
germinal  variations  than  members  of  the  same  species 
having  the  diploid  number  of  chromosomes,  because 
variations  of  all  kinds  would  be  recognizable  in  the 
former  case,  while  in  the  latter  recessive  variations  could 
not  be  detected  until  the  first  or  second  filial  generation 


EVOLUTION  199 

and  then  only  when  the  proper  mating  was  made.  Though 
there  is  no  direct  support  for  this  idea  in  the  species  where 
the  premises  hold,  there  is  some  evidence  that  the  reason- 
ing is  not  wholly  improbable.  Bud-variations  occur  much 
more  frequently  in  heterozygotes  than  in  homozygotes. 
This  simply  means  that  bud-variations  are  brought  to 
light  more  frequently  in  heterozygotes  than  in  homozy- 
gotes :  and  a  reason  is  not  hard  to  find.  Recessive  varia- 
tions are  much  more  frequent  than  dominant  variations, 
and  a  recessive  variation  in  a  particular  character  shows 
only  when  the  organism  is  heterozygous  for  that  char- 
acter. If  a  recessive  bud-variation  arises  in  a  homozygote 
and  gametes  are  afterwards  developed  from  the  sporting 
branch,  it  is  not  at  all  unlikely  that  the  variation  may 
show  in  the  next  generation,  but  it  will  be  attributed  then 
to  gametic  mutation. 

If,  therefore,  one  is  constrained  to  admit  that  the  pre- 
ponderance of  the  evidence  points  to  practically  the  same 
coefficient  of  heredity  for  both  forms  of  reproduction,  and 
that  variation  in  the  sense  of  actual  changes  in  germinal 
constitution  may  occur  with  greater  frequency  in  asexual 
reproduction,  if  there  be  any  difference  at  all  between 
the  two  forms,  he  is  left  with  only  one  reasonable  hypoth- 
esis to  account  for  everything,  Mendelian  segregation 
and  recombination. 

Mendelian  heredity  is  a  manifestation  of  sexual  re- 
production. Wherever  it  occurs,  there  Mendelian  heredity 
will  be  found.  Now  if  N  variations  occur  in  the  germ- 
plasm  of  an  asexually  reproducing  organism,  only  N  types 
can  be  formed  to  offer  raw  material  to  selective  agencies. 
But  if  N  variations  occur  in  the  germ  plasm  of  a  sexually 
reproducing  organism  2"  types  can  be  formed.    The  ad- 


200         INBREEDING  AND  OUTBREEDING 

vantage  is  almost  incalculable.  Ten  variations  in  an 
asexual  species  mean  simply  10  types ;  10  variations  in  a 
sexual  species  mean  the  possibility  of  1024  types.  Twenty 
variations  in  the  one  case  is  again  only  20  types  to  survive 
or  perish  in  the  struggle  for  existence ;  20  variations,  in 
the  other  case,  may  present  1,032,576  types  to  compete  in 
the  struggle.  It  is  necessary  to  condition  the  argument 
by  pointing  out  that  these  figures  are  the  maximum  possi- 
bilities in  favor  of  sexual  reproduction.  It  is  improbable 
that  they  ever  actually  occur  in  nature,  for  2^^  types  really 
to  be  found  in  the  wild  competing  for  place  after  only  20 
germinal  variations  would  mean  an  enormous  number  of 
individuals  even  if  the  20  changes  had  taken  place  in  dif- 
ferent chromosomes,  and  if  the  variations  were  linked  at 
all  closely  in  inheritance  the  number  required  would  be 
staggering.  But  there  are  breaks  in  linked  inheritance, 
and  the  possibility  is  as  stated.  Associated  with  this 
benefit  arising  from  the  law  of  recombination,  there  is 
another  of  great  practical  importance  resulting  from  the 
phenomenon  of  dominance.  Recessive  variations  may 
arise,  which  in  the  particular  factorial  complex  existing 
in  the  individuals  at  the  time  of  origin,  would  cause  the 
possessors  to  be  eliminated  by  natural  selection.  These 
variations,  however,  may  be  carried  an  indefinite  number 
of  generations  in  the  heterozygous  condition,  thus  multi- 
plying the  chances  that  they  finally  be  combined  with 
other  factors  in  complexes  which  as  a  whole  are  desirable. 
These  inestimable  advantages  remain  even  though  it 
should  be  shown  later  that  the  more  fundamental  and 
generalized  characters  of  an  organism  are  not  distributed 
by  Mendelian  heredity.  Loeb  ^^9  suggests  that  the  cyto- 
plasm of  the  egg  is  roughly  the  potential  embryo  and  that 


EVOLUTION  201 

the  chromosomes,  distributed  as  required  by  the  breeding 
facts  of  Mendelian  heredity,  are  the  machinery  for  im- 
pressing the  finer  details.  There  is  very  little  to  be  said 
for  this  point  of  view,  though  it  may  have  use  as  a  working 
hypothesis.  But  granting  its  truth,  it  does  not  detract 
from  the  benefits  gained  by  the  origin  of  sex ;  the  major- 
ity of  variations  are  comparatively  small,  changes  in 
detail,  the  very  kind  which  are  known  to  be  Mendelian 
in  their  inheritance. 

Yet  sexual  reproduction  in  itself  does  not  assure  these 

I  advantages,  though  they  are  based  upon  it.    There  must 

he  means  for  the  mixture  of  germ  plasms.  This  oppor- 
tunity was  furnished  originally  by  bisexuality.  After- 
wards hermaphroditism  was  tried ;  and,  though  manifestly 
an  economic  gain,  it  was,  on  the  whole,  unsuccessful  except 

g  as  functional  bisexuality  was  restored  by  self-sterility, 
protandry,  protogyny  or  mechanical  devices  which  pro- 
moted cross-fertilization.    The  prime  reason  for  the  suc- 

1  cess  of  sexual  reproduction,  then,  as  Weismann  209  first 
maintained,  though  he  knew  not  the  exact  reason,  is  the 
opportunity  it  gives  for  mingling  germ  plasms  of  different 

I  constitutions,  and  thereby  furnishing  selective  agencies 
many  times  the  raw  material  producible  through  asexual 
reproduction.  It  was  not  sexual  reproduction  per  se 
which  triumphed,  but  exogamy. 

While  increased  variability  and  the  greater  elasticity 
in  adaptiveness  to  new  environments  thus  gained  must  be 
given  the  first  consideration  when  seeking  the  significance 
of  sex,  they  are  not  the  only  advantages.  As  we  have 
seen,  an  increased  size,  greater  viability  and  increased 
production  of  offspring  commonly  result  from  crossing 
somewhat  different  forms.     Here  is  a  combination  of 


202         INBKEEDING  AND  OUTBREEDING 

qualities  unquestionably  having  survival  value  in  the 
great  majority  of  oases.  It  is  a  phenomenon  so  universal, 
so  uniform  in  its  effect,  it  must  have  played  an  important 
role  during  the  course  of  evolution.  Heterosis  increasing 
growth  and  fertility  immediately,  segregation  favoring 
adaptibility  in  the  next  generation,  is  a  partnership  of 
some  strength.  An  income  for  life  and  a  trust  fund  ma- 
turing for  benefit  of  the  children,  what  more  could 
one  ask? 

Heterosis  may  even  be  pictured  as  the  efficient  cause  of 
sex  survival.  Some  means  of  favoring  union  of  dissimilar 
spores  occurred  as  a  chance  variation.  Through  the  com- 
bination of  somewhat  different  qualities  this  new  dual 
product,  the  zygote,  was  better  enabled  to  develop  and  to 
reproduce.  Its  survival  coefficient  was  high.  The  ten- 
dency for  union  of  spores  persisted  and  became  char- 
acteristic of  the  species.    Sex  was  established. 

This  is  a  pleasing  theoretical  picture,  and  we  do  not 
believe  it  is  overdrawn,  but  it  must  be  admitted  that  the 
concrete  evidence  of  a  sexual  union  being  immediately 
beneficial  in  the  lower  organisms  is  not  what  might  be 
desired.  Jennings  ^^^  finds  a  marked  slowing  down  of 
the  reproductive  rate  in  the  generations  immediately  fol- 
lowing conjugation  in  Paramecium,  with  no  beneficial 
effect  resembling  heterosis,  although  he  suggests  that  re- 
combination may  account  for  the  best  cultures.  In  fact, 
nothing  similar  to  heterosis  has  been  found  in  unicellular 
organisms.  The  lowest  tjipe  where  distinct  evidence  of 
the  phenomenon  has  been  discovered  is  in  Trochelminthes, 
cross-fertilization  increasing  size,  vigor,  viability  and  re- 
productive rate  in  rotifers.  But  it  would  be  strange 
indeed  if  no  such  effect  did  occur  in  low  forms  when  it  is 


Fig.  40. 


M   ^       & 


i^Mi^iJi 


Fig  40. — Ears   of  a   first   generation   cross   between   two  inbred  strains  of  maize  showing 

uniformity. 
Fig.  41. — Plants  of  the  first,  generation  cross  between  the  sami'  twi>  inbred  strains  <>f  maiip. 

Note  the  uniformity  in  lieight. 


■!*  Vv  -^sXtfifc^^v  X4!i* 


Fig.  42. — Diagram  showing  a  method  of  double  crossing  maize  to  secure  maximum  yields, 

illustrated  by  actual  field  results. 


EVOLUTION  203 

so  widespread  in  all  the  higher  plants  and  animals.  Herit- 
able variations  are  constantly  arising  in  simple  organisms, 
as  has  been  demonstrated  by  J  ennings  ^^"^  in  Difflugia,  and 
it  may  be  assumed  that  these  are  in  part  favorable  and  in 
part  unfavorable.  The  union  of  two  individuals  would 
have  the  same  chance  of  bringing  together  the  greatest 
number  of  favorable  growth  factors  and  the  progeny 
would  thus  be  benefited,  even  though  the  mechanism  fur 
bringing  this  about  is  not  as  well  organized  as  in  the 
higher  forms. 

Some  evidence  of  the  possible  importance  of  heterosis 
in  the  establishment  of  sex  may  be  obtained  by  the  con- 
sideration of  an  analogous  phenomenon,  double  fertiliza- 
tion among  the  angiosperms.  In  the  gymnosperms  the 
embryo  develops  from  the  fertilized  germ  cell,  of  course, 
but  the  endosperm  which  nourishes  the  young  seedling  is 
gametophyte  tissue.  In  the  angiosperms  the  endospenn 
as  well  as  the  embryo  develops  after  a  fertilization  has 
taken  place.  The  conditions  are  slightly  different,  as  a 
fusion  between  two  maternal  nuclei  occurs  before  tin? 
union  with  the  second  male  nucleus,  but  the  essential 
feature  is  the  same  as  in  the  production  of  the  embryo — 
different  hereditary  materials  are  united  when  cross-fer- 
tilization occurs.  And  in  the  same  way  that  the  embr^^o 
and  the  resulting  plant  may  be  greatly  benefited  by  cross- 
fertdization,  so  also  is  the  endosperm  tissue  increased  in 
amount  as  a  direct  manifestation  of  hybrid  vigor. 

Nemec^^2  j^^g  sought  to  account  for  endosperm  hy- 
bridization as  an  adaptation  which  results  in  a  better 
adjustment  of  the  composition  of  the  reserve  food  supply 
to  the  needs  of  a  hybrid  embryo.  The  cross  between  some- 
what different  types  results  in  an  embryo  which  presum- 


204         INBEEEDING  AND  OUTBEEEDING 

ably  partakes  of  certain  features  of  both  parents.  If 
forced  to  depend  upon  food  supplied  by  only  one  parent  it 
might  be  handicapped  to  some  extent  in  comparison  with 
another  embryo  supplied  with  food  which  was  intermedi- 
ate with  respect  to  the  two  parents.  If  endosperm  hybrid- 
ization does  indeed  supply  such  a  need,  the  fact  that  the 
endosperm  is  also  increased  in  amount  would  have  equal 
importance.  It  may  well  be  that  to  fill  either  purpose 
endosperm  hybridization  has  sufficient  value  to  account 
for  its  maintenance  in  the  angiosperms.  However  this 
may  be,  increased  adaptability  through  recombination  of 
characters  which  is  such  an  important  factor  in  sexual 
reproduction  has  no  significance  in  this  case,  as  the  endo- 
sperm does  not  perpetuate  itself. 

Additional  light  may  be  thrown  on  the  importance  of 
heterosis  in  sex  origin  from  the  part  it  possibly  has  had  in 
a  related  series  of  events.  In  the  algae  and  mosses,  the 
principal  life  processes  are  carried  on  in  the  haploid  gen- 
eration and  the  parts  which  result  from  fertilization  and 
produce  the  spores  are  relatively  insignificant  and  are 
dependent  upon  the  gametophyte  for  maintenance.  How 
the  sporophyte  has  gradually  become  more  specialized, 
taking  up  the  manufacture  of  food  for  itself  until  finally 
the  relations  are  changed  completely,  are  matters  of 
common  knowledge.  This  series  of  events  is  usually 
referred  to  as  the  rise  of  the  sporophyte  and  decline  of 
the  gametophyte. 

Just  why  there  has  been  this  radical  and  complete 
change  in  the  plant  kingdom  is  rather  difficult  to  explain, 
but  it  should  be  noted  that  the  increased  variability  and 
greater  adaptability  which  seems  reasonable  in  account- 
ing in  a  large  measure  for  the  survival  of  sex,  is  not 


EVOLUTION  205 

applicable  here.  Recombinations  occurring  at  the  reduc- 
tion division  can  be  utilized  by  the  gametophyte  as  well  as 
by  the  sporophyte,  hence  there  seems  to  be  no  necessity 
for  plants  to  change  from  dominant  gametophytes  to 
dominant  sporophytes  in  order  to  secure  the  greater 
adaptability  offered  by  sexual  reproduction.  Every  com- 
bination of  characters  possible  in  the  sporophyte  occurs 
in  the  haploid  condition,  if  we  leave  out  of  considera- 
tion heterozygous  combinations  which  are  the  interaction 
of  two  members  of  a  contrasted  pair  of  factors  and  can- 
not be  fixed.  Haploid  combinations  even  have  certain 
advantages  over  diploid  combinations  in  that  all  the  char- 
acters are  expressed  and  offer  more  material  to  natural 
selection.  Favorable  combinations  have  a  better  chance 
of  survival  and  unfavorable  combinations  are  more 
quickly  eliminated.  If,  then,  variability  is  not  a  factor 
in  the  rise  of  the  sporophyte,  and  if  we  refuse  to  admit 
any  value  in  chromosome-doubling  itself,  and  the  evidence 
certainly  does  not  indicate  that  it  has  any  significance,  the 
only  factor  which  remains,  as  far  as  we  can  see  now,  is  the 
vigor  derived  from  hybridization.  Heterosis,  of  course, 
can  operate  only  in  the  sporophyte.  In  the  lower  plants 
where  the  sporophyte  is  less  important  in  the  life  cycle, 
heterosis  would  be  of  value  only  in  spore  formation.  Later 
as  the  sporophyte  became  of  more  consequence,  heterosis 
would  have  had  more  and  more  value ;  and  it  may  well  be 
that  it  had  considerable  to  do  with  this  revolution  in 
plant  life. 

If  sexual  reproduction  is  so  useful  that  it  has  been 
adopted  as  the  principal  means  of  reproduction  at  a 
sacrifice  of  speed  of  multiplication  and  economy  of  ma- 
terial, why,  then,  has  it  been  given  up  by  the  many  species 


206         INBEEEDING  AND  OUTBEEEDING 

which  have  resorted  to  vegetative  propagation  or  par- 
thenogenesis? Even  self-fertilization,  which  is  the  rule 
with  many  plants,  nullifies  the  advantages  which  were  re- 
sponsible for  its  development.  As  far  as  there  is  signifi- 
cance in  amphimixis  in  inducing  variability,  continuous 
self-fertilization  must  for  the  most  part  be  left  out  of 
consideration.    "Weismann  ^^^  states  the  problem : 

If  amphimixis  has  been  abandoned  in  the  course  of  phylogeny  by 
isolated  groups  of  organisms,  this  has  happened  beoause  other  advan- 
tages accrued  to  them  in  consequence,  which  gave  them  greater  security 
in  the  struggle  for  existence ;  but  it  must  be  admitted  that  they  thereby 
lost  their  perfect  power  of  adaptation,  and  that  they  have  thus  bartered 
their  future  for  the  temporary  securing  of  their  existence. 

Let  us  see  what  it  is  for  which  these  organisms  ^  *  barter 
their  future.''  According  to  the  view  of  heterosis  out- 
lined previously,  there  is  no  advantage  in  the  hetero- 
zygous state  in  itself,  but  on  account  of  linkage  it  is 
difficult  to  obtain  all  the  more  favorable  characters  which 
exist  in  a  species  combined  in  one  individual  in  a  pure 
breeding,  homozygous  condition.  There  is  always  the 
possibility  of  obtaining  such  combinations,  however,  and 
the  resulting  individuals  are  well  fitted  for  survival 
as  long  as  the  environment  remains  the  same.  If  the  pro- 
duction of  these  favored  few  is  accompanied  by  any 
change  which  renders  cross-fertilization  difficult,  and  if 
there  is  nothing  to  prevent  them  from  resorting  to  self- 
fertilization,  parthenogenesis  or  vegetative  means  of 
propagation,  there  is  no  obvious  reason  why  the  plants 
should  not  undergo  the  change.  They  would  possess  the 
most  efficient  means  of  multiplication  and  would  doubtless 
be  fitted  for  survival  through  long  periods  of  time.  They 
would  not  be  flexible,  however,  and  if  the  environment 


EVOLUTION  207 

changed  would  probably  lose  in  the  race  with  more  adapt- 
able cross-fertilized  forms.  Their  handicap  is  their  lack 
of  chances  for  progress. 

A  seoondaiy  advantage  of  sexual  reproduction  is  the 
division  of  labor  made  possible  by  secondary  sexual  char- 
acters, using  the  term  very  generally  and  including  even 
such  differences  as  those  which  separate  the  egg  and  the 
sperm.  It  is  not  known  just  how  these  differences  arose 
or  by  what  mechanism  they  are  transmitted.  The  great- 
est hope  of  reading  the  riddle  lies  in  an  investigation  of 
hermaphroditic  plants,  for  there  are  technical  difficulties 
which  till  now  have  prevented  its  solution  in  animals.  For 
example,  breaks  in  the  linkage  between  sex-linked  char- 
acters occur  only  in  the  female  in  Drosophila,  and  as  the 
sex  chromosome  is  double  in  the  female,  it  cannot  be  de- 
termined whether  the  differentiation  between  male  and 
female  is  due  to  the  whole  chromosome  or  not.  But  this 
ignorance  does  not  give  reason  for  a  denial  of  the 
great  advantage  which  sexes  bearing  different  characters 
hold  over  sexes  alike  in  all  characters  except  the  primary 
sex  organs. 

The  only  glimpse  of  the  truth  we  have  on  the  matter 
comes  from  recent  work  on  the  effect  of  secretions  of  the 
sex  organs  on  secondary  sexual  characters.  The  effect  of 
removing  the  sex  organs  and  the  result  of  transplanting 
them  to  abnormal  positions  in  the  body  have  sho\\Ti  that 
in  vertebrates  the  secretions  of  these  organs  themselves 
activate  the  production  of  the  secondary  sexual  char- 
acters. This  does  not  seem  to  be  the  case  in  arthropods, 
however;  so  one  cannot  say  that  primary  sexual  differ- 
entiation and  secondary  sexual  differentiation  are  one  and 
the  same  thing.    Nevertheless,  the  generalization  is  not 


208         INBEEEDING  AND  OUTBEEEDING 

improbable.  Surgical  castrations  of  insects  which  have 
not  altected  the  secondary  sexual  characters  even  though 
made  in  the  early  larval  stages,  are  not  conclusive  because 
of  the  possibility  of  the  primary  sex  organs  having  a 
marked  influence  on  development  very  early  in  the  life 
cycle ;  and  parasitic  castration  brings  in  another  variable 
through  the  presence  of  the  alien  organism. 

Again,  there  is  a  presumable  advantage  in  bisexual 
reproduction  in  having  sex-linked  characters.  We  say 
presumable  advantage,  for  all  of  the  relationships  be- 
tween sex  and  sex-linked  characters  are  not  clear.  The 
facts  are  these :  One  sex  is  always  heterozygous  for  the 
sex  determiner  and  the  factors  linked  with  it.  Even  if 
there  be  no  actual  advantage  in  the  heterozygous  condi- 
tion, if  heterosis  prove  to  be  only  an  expression  of  the 
meeting  of  dominant  characters,  a  possible  advantage  still 
accrues  to  this  phenomenon  because  the  mechanism  con- 
tributes toward  mixing  of  germ  plasms.  As  an  example, 
let  us  take  the  Drosophila  type  of  sex  determination. 
There  the  sperm  is  of  two  kinds ;  the  one  containing  the 
sex  chromosome  and  its  sex-linked  factors,  the  other  lack- 
ing it.  The  eggs  are  all  alike,  each  bearing  the  sex 
chromosome.  It  follows,  then,  that  the  male  always  re- 
ceives this  chromosome  from  his  mother  who  may  have 
received  it  from  either  her  father  or  mother.  Moreover, 
further  variability  may  be  derived  from  the  linkage 
breaks  which  occur  always  in  the  female.  This  last  phe- 
nomenon is  hardly  worthy  of  special  mention,  however, 
until  it  is  shown  to  be  typical  of  bisexual  reproduction. 

This  short  reconnaissance  presents  only  the  facts  on 
the  role  of  reproduction  in  evolution  as  they  are  affected 
directly  or  indirectly  by  inbreeding  and  outbreeding.    A 


EVOLUTION  209 

very  great  number  of  interesting  things  connected  with 
reproduction  during  the  course  of  evolution  have  not  been 
mentioned.  This  is  because  it  is  felt  that  the  vital  feature 
in  the  whole  affair,  the  persistence  in  both  the  animal  and 
the  plant  kingdoms  of  innumerable  mechanisms  providing 
for  cross-fertilizations,  is  to  be  explained  solely  on  the 
ground  of  offering  selective  agencies  the  greatest  amount 
of  raw  material.  Mendelian  recombination  is  thus 
assigned  a  part  in  phylogenetic  development  second  only 
to  inherent  variability,  and  the  whole  history  of  repro- 
ductive change  becomes  clear  without  the  ill-advised 
assumption  that  complex  processes  like  autogamy  are 
harmful  in  themselves. 


14 


CHAPTER  XI 

THE  VALUE  OF  INBEEEDING  AND  OUTBREED- 
ING  IN  PLANT  AND  ANIMAL  IMPROVEMENT 

The  origin  of  our  more  important  domestic  animals 
and  cultivated  plants  is  a  matter  on  wliich  there  is  no 
direct  evidence.  Among  animals  the  ostrich  is  the  only 
example  of  modern  domestication;  among  plants  not  a 
single  species  of  great  economic  worth  has  been  brought 
into  cultivation  within  historic  times.  If  one  must  have 
a  theory  concerning  their  genesis,  and  what  one  of  us 
does  not  delight  in  theorizing,  the  weight  of  evidence  is  in 
favor  of  a  poly-phyletic  origin  in  nearly  every  case.  There 
is  more  than  one  ivild  species  related  to  our  modern  dogs, 
cattle,  swine  and  sheep,  our  wheats,  barleys,  apples  and 
grapes;  and  these  species  will  cross  together  and  yield 
partially  fertile  hybrids.  The  wild  relatives  of  the  do- 
mestic forms  were  variable,  so  variable  that  many  species 
were  differentiated  by  natural  causes;  yet  these  species 
groups  remained  so  well  adapted  to  each  other  germinally 
that  their  hybrids  are  not  completely  sterile.  What  seems 
more  reasonable  than  to  suppose  the  original  domestic 
races  to  have  been  produced  by  uniting  two  or  more  wild 
types  and  following  this  union  of  diverse  germ  plasms 
with  more  or  less  close  inbreeding  and  selection? 

Such  procedure,  at  least,  has  been  the  method  whereby 
the  clearly  distinct  and  highly  valuable  breeds  of  the  pres- 
ent day  have  originated.  Take  the  draft  horses  as  an 
example.  In  the  early  days  of  Europe  native  breeds  were 
developed  in  every  country  for  military  purposes.    Just 

210 


PLANT  AND  ANIMAL  IMPROVEMENT       211 

how  tliey  originated  we  cannot  say.  The  obvious  fact  is 
that  none  of  them  developed  outstanding  merits  except 
the  Flemish  horse.  Then  improvement  became  rapid  and 
steady.  With  an  infusion  of  Flemish  blood  came  the 
remarkable  development  of  the  Clydesdale  in  Scothand, 
the  Shire  in  England,  and  the  Belgian  in  the  low  countries. 
Adding  the  Arabian  blood  which  came  in  with  the  defeat 
of  the  Saracens  in  732,  and  the  wonderful  Percheron  of 
France  came  into  being.'^^ 

Similarly  the  origin  of  all  modem  breeds  of  coach, 
light  harness  and  saddle  horses  may  be  traced.  To  the 
native  breeds  of  Europe  were  added  the  blood  of  tlie 
iBarb  or  its  derivatives,  the  Turk  and  the  Arabian.  In 
France,  in  Spain,  in  England  and  in  Russia  the  history  is 
the  same — hybridization,  then  close  breeding  and  selection. 

If  one  turns  to  cattle,  the  story  varies  but  little.  The 
basis  of  our  modern  strains  is  the  cross  between  domesti- 
cated progeny  of  wild  European  cattle  and  their  Asiatic 
relatives.  From  this  stock  numerous  breeds  grew  up  dif- 
fering in  contour,  size  and  color.  Some  were  horned, 
others  were  hornless.  Some  were  developed  for  meat  pro- 
duction, large  at  maturity  and  quick  in  attaining  it ;  others 
were  selected  for  the  dairy,  a  great  milk  production  and  a 
high  percentage  of  butter  fat.  As  time  went  on  and  com- 
mercial channels  became  better  established  crosses  were 
made  between  the  better  animals  of  the  different  beef 
breeds  and  between  those  of  the  various  dairy  breeds. 
Crossing  followed  by  inbreeding  has  been  the  touchstone 
of  success. 

Similar  more  or  less  useless  generalities  could  be  given 
about  swine,  sheep,  dogs,  cats,  the  cereals,  the  perennial 
fruits,  the  numerous  floricultural  novelties,  but  this  would 


212         INBEEEDING  AND  OUTBEEEDING 

serve  no  purpose.  We  have  seen  from  our  consideration 
of  the  facts  of  heredity  that  both  inbreeding  and  out- 
breeding must  be  used  if  one  would  succeed  in  improving 
the  products  of  domestication.  There  must  be  cross-breed- 
ing to  furnish  a  variety  of  character  combinations  from 
which  to  select;  there  must  be  inbreeding  to  provide  the 
opportunity  to  isolate  the  combinations  desired.  What 
we  want  to  know  now  is  the  manner  of  their  use,  the  degree 
of  inbreeding  permissible  under  given  conditions,  the  effi- 
cacy of  crossing  for  particular  purposes. 

While  there  has  always  been  a  certain  amount  of  in- 
breeding as  a  necessary  adjunct  in  building  up  breeds  of 
livestock  because  of  the  necessity  of  mating  near  relatives 
in  order  to  establish  uniformity,  the  opinions  of  breeders 
have  differed  and  still  differ  as  to  how  long  or  how  close 
intermating  can  be  practiced  with  safety.  Yet  some  of 
the  most  noted  modern  livestock  strains  owe  their  excel- 
lence to  a  close  and  continuous  inbreeding  that  would  be 
looked  upon  with  misgivings  by  the  majority  of  animal 
raisers.  In  fact,  some  of  the  inbreeding  actually  prac- 
ticed was  due  more  to  enforced  isolation,  or  the  expense 
or  difiiculty  of  securing  unrelated  animals  with  desirable 
characteristics,  than  to  a  firm  belief  in  the  desirability  of 
the  method.  This  might  be  said  of  the  Shetland  pony,  the 
Angora  goat,  the  Merino  sheep  in  America,  and  of  many 
breeds  of  dogs. 

Notwithstanding  these  facts,  it  would  be  a  mistake  not 
to  recognize  how  great  an  amount  of  continuous  and  ex- 
tended inbreeding  has  been  practiced  intentionally  with 
the  best  of  results  after  the  general  characteristics  of  a 
breed  have  been  established.  This  is  true  as  a  generalized 
statement  for  the  modern  trotting  horse  and  saddle  horse 


Fig.  43. — First  generation  cross  resulting  from  a  pure  bred  Slirojisliiic  ram  mainl 
with  grade  Delaine-Merino  ewes.  First  prize  cross-bred  yearling  ewes  at  tin-  hitcrnatiniial 
Livestock  Exposition  in  1917.     (From  Severson.) 


PLANT  AND  ANIMAL  IMPROVEMENT        213 

which  have  shown  so  much  speed;  for  the  Shorthorn  and 
Hereford,  the  most  famous  English  breeds  of  beef  cattle; 
for  the  Southdown  and  the  other  famous  sheep  breeds,  the 
Shropshire,  the  Oxford  and  the  Hampshire,  to  which  it 
has  given  rise;  and  for  almost  all  of  the  more  famous 
breeds  of  dogs,  not  even  excepting  the  large  types,  the 
mastiff,  the  St.  Bernard,  and  the  Newfoundland,  which 
are  derived  from  the  Tibetan  dog,  Canis  niger,  as  a  foun- 
dation stock. 

Perhaps  the  most  notable  examples  of  conscious  use 
of  intense  inbreeding  in  developing  breeds  of  marked  ex- 
cellence are  the  dairy  cattle  of  the  channel  islands,  the 
Jersey  and  the  Guernsey.  One  does  not  need  to  describe 
or  to  eulogize  these  strains.  What  they  are  and  what  they 
have  accomplished  in  producing  milk  and  butter  fat  are 
known  throughout  the  world.  Starting  with  the  cattle  of 
Normandy  and  Brittany  as  foundation  stock,  these  two 
breeds  have  been  built  up  by  persistent  use  of  a  more 
intense  system  of  inbreeding  than  is  recorded  in  the  his- 
tory of  any  other  strain  of  livestock.  In  fact,  since  1763 
rigidly  enforced  laws  have  prevented  landing  any  live 
cattle  whatsoever  on  either  island  except  for  slaughter. 
When  one  realizes  that  the  larger  of  these  two  islands, 
that  of  Jersey,  is  but  eleven  miles  long  by  six  miles  wide, 
he  can  appreciate  the  amount  of  inbreeding  these  laws 
have  promoted. 

With  swine,  one  gathers  that  injurious  results  from 
close  mating  may  be  somewhat  more  pronounced  than 
with  some  other  animals ;  in  other  words,  that  swine  carry 
a  large  number  of  deleterious  recessive  characters.  But 
many  of  the  famous  breeds  of  swine  have  been  rather 
closely  inbred.    Mr.  N.  H.  Grentry  of  Sedalia,  Missouri, 


\ 


214         INBREEDING  AND  OUTBREEDING 

who  has  achieved  quite  a  remarkable  success  with  Berk- 
shires,  rarely  went  outside  of  his  own  drove  for  breeding 
stock.  He  is  quoted  as  saying  (Mumford  ^^^) :  "If  it  is 
true  that  inbreeding  intensifies  weakness  of  constitution, 
lack  of  vigor,  or  too  great  fineness  of  bone,  as  we  all  be- 
lieve, is  it  not  as  reasonable  and  as  certain  that  you  can 
intensify  strength  of  constitution,  heavy  bones,  or  vigor, 
if  you  have  these  traits  w^ell  developed  in  the  blood  of  the 
animals  you  are  inbreeding?  I  think  I  have  continued  to 
improve  my  herd,  being  now  able  to  produce  a  larger  per- 
centage of  really  superior  animals  than  at  any  time  in 
the  pasf 

This  quotation  exemplifies  the  opinion  of  the  best  in- 
formed of  the  practical  breeders  of  the  present  day  in  re- 
gard to  the  practice  of  inbreeding.  In  general  they  recog- 
nize that  the  results  obtained  depend  largely  upon  the 
character  and  constitution  of  the  animals,  and  the  care 
and  skill  with  which  they  are  selected  for  mating.  They 
have  learned  by  experience  what  matings  are  the  most 
successful  and  how  far  it  is  advisable  to  carry  close  breed- 
ing with  a  particular  stock.  Rarely  is  inbreeding  as  close 
as  brother  and  sister  or  parent  and  offspring  mating  con- 
tinued for  many  successive  generations,  however ;  for  they 
are  apprehensive  at  all  times  that  inbreeding  may  reduce 
the  fertility  and  lessen  the  constitutional  vigor  of  their 
animals,  and  they  frequently  introduce  stock  from  outside 
to  counteract  any  tendency  in  this  direction  whether 
fancied  or  real. 

In  plants  the  problem  is  different.  No  systematic  in- 
dividual mating  system  is  practiced,  as  is  the  case  with 
animals,  so  that  whether  plants  are  inbred  or  outbred  is  a 
matter  w^hich  is  left  to   regulate   itself   automatically. 


PLANT  AND  ANIMAL  IMPROVEMENT        215 

Among  those  plants  which  are  largely  self-pollinated  by 
nature,  chance  crossing,  or,  in  some  cases,  systematic 
hybridization,  has  originated  new  types.  Self-pollination 
has  brought  these  types  to  uniformity,  and  by  isolation 
new  varieties  have  been  established.  Among  naturally 
crossed  plants  genetical  variations  are  continually  being 
produced  and  selection  for  certain  of  the  more  conspicu- 
ous features  has  led  to  the  creation  of  well-marked  varie- 
ties. Indian  com  is  one  of  the  best  examples  in  this  class. 
There  are  many  distinct  types,  and  the  less  distinct  but 
fairly  well  recognized  varieties  are  almost  innumeral)lo, 
adapting  the  plant  to  a  range  of  conditions  from  the  edge 
of  the  Arctics  to  the  Tropics,  throughout  the  world. 

In  eveiy  locality  where  com  is  grown  the  usual  habit 
is  to  prevent  inbreeding  as  much  as  possible.  Many  com 
growers  make  a  regular  practice  of  bringing  in  seed  from 
other  localities,  and  often  two  or  more  somewhat  different 
varieties  are  planted  together  and  allowed  to  mix.  The 
reason  why  this  practice  is  followed  is  easily  apparent 
from  the  controlled  experiments  on  the  effects  of  inbreed- 
ing and  cross-breeding  upon  this  plant.  But  even  keeping 
in  mind  the  injurious  results  of  inbreeding,  indiscriminate 
crossing  is  not  desirable.  Many  of  the  well-known  varie- 
ties in  the  Com  Belt  States,  such  as  Eeid^s  Yellow  Dent, 
Learning,  and  Boone  County  White,  are  the  results  of 
long-continued  selection  for  certain  standards  without 
crossing  with  other  varieties.  Inbreeding,  therefore,  has 
secured  individuality  for  varieties  of  cultivated  plants  as 
well  as  for  breeds  of  animals. 

The  value  of  inbreeding  in  plant  and  animal  improve- 
ment in  the  past  may  be  summed  up  in  the  statement  that 
it  is  the  greatest  single  agency  in  bringing  about  uni- 


216         INBEEEDING  AND  OUTBKEEDING 

formity  and  the  concentration  of  desired  qualities.  So 
valuable  have  been  the  results,  particularly  with  animals, 
that  it  has  often  been  continued  even  though  concentra- 
tion of  characters  which  made  for  lessened  constitutional 
vigor  and  fertility  accompanied  the  accumulation  of  de- 
sirable features,  for  the  good  outweighed  the  evil.  To 
overcome  anticipated  calamities,  animal  breeders  have 
from  time  to  time  introduced  fresh  stock.  In  doing  this 
they  certainly  were  wise,  since  a  rather  high  probability 
always  exists  that  such  a  procedure  will  introduce  the 
dominant  complements  of  the  detrimental  characters. 
But  even  granting  the  good  sense  at  the  base  of  both  prac- 
tices, it  may  be  doubted  whether  inbreeding  and  cross- 
breeding have  been  used  in  the  best  possible  manner  as 
means  of  improvement.  There  are  precise  uses  to  which 
each  may  be  put  which  hitherto  have  not  been  considered. 
Experiments  with  maize  show  that  undesirable  quali- 
ties are  brought  to  light  by  self-fertilization  which  either 
eliminate  themselves  or  can  be  rejected  by  selection.  The 
final  result  is  a  number  of  distinct  types  which  are  con- 
stant and  uniform  and  able  to  persist  indefinitely.  They 
have  gone  through  a  process  of  purification  such  that  only 
those  individuals  which  possess  much  of  the  best  that  was 
in  the  variety  at  the  beginning  can  survive.  Although 
these  resultant,  purified  t^^es  have  little  value  in  them- 
selves, they  have  possibilities.  The  characters  which  they 
have  can  now  be  estimated  more  nearly  at  their  true 
worth.  By  crossing,  the  best  qualities  which  have  been 
distributed  to  the  several  inbred  strains  can  be  gath- 
ered together  again  and  a  new  variety  re-created.  Size, 
vigor  and  fertility  can  be  fully  restored  with  the  advan- 


•■>i>T-,'CC.*S 


Fig.  44. — "California  Favorite,"  finst  generation  cross  between  a  Hereford  and  Sliort- 
horn;  grand  champion  steer  at  the  International  Livestock  Exposition  in  1910;  considered 
to  be  one  of  the  finest  steers  ever  exhibited.     (.From  True.) 


i 


PLANT  AND  ANIMAL  IMPROVEMENT        217 

tage  of  real  improvement  through  the  elimination  of 
certain  undesirable  characters. 

At  present,  this  application  of  inbreeding  to  the  im- 
provement of  cross-bred  animals  and  plants  is  somewhat 
of  an  unknown  quantity.  It  has  not  been  as  thoroughly 
tested  as  might  be  desired,  but  the  basic  principle  is 
sound.  Although  it  is  a  drastic  procedure,  it  is  merely 
utilizing  to  the  fullest  extent  what  practical  breeders 
have  recognized  as  one  of  the  most  valuable  benefits  of 
close  mating.  Accepting  the  doctrine  that  consanguinity 
in  itself  is  not  in  any  way  injurious  and  that  good  or  evil 
results  from  it  solely  through  the  inheritance  received, 
we  can  attack  the  century-old  problem  of  inbreeding  with 
a  clarity  of  vision  heretofore  impossible.  Breeds  of  ani- 
mals, and  naturally  crossed  varieties  of  plants,  which  are 
necessarily  more  or  less  heterogeneous  in  their  hereditary 
constitution,  can  be  split  up  into  their  component  parts 
by  this  means.  The  pure  types  obtained  can  then  be 
selected  wdth  far  more  surety  than  is  ever  possible  with 
organisms  in  a  continuously  hybrid  condition,  thereby 
presenting  basic  stock  of  tested  value  for  further  hybrid- 
ization and  recombination. 

With  plants  the  application  of  this  method  would  be 
simpler  than  with  animals.  Most  naturally  crossed  plants 
can  be  artificially  self -fertilized  and  constancy  and  uni- 
formity reached  in  about  eight  generations  if  there  are 
no  complicating  factors  such  as  self -sterility.  The  ex- 
pense would  not  be  prohibitive,  although  many  pure  lines 
must  be  tested  in  order  to  have  a  high  probability  of 
obtaining  all  that  is  best  in  a  variety.  After  the  most 
desirable  combinations  are  isolated,  their  recombination 
into  a  new  and  better  varietv,  which  could  be  maintained 


218         INBEEEDING  AND  OUTBREEDING 

by  seed  propagation,  would  be  a  comparatively  easy 
undertaking. 

"With  plants  which  are  propagated  vegetatively,  the 
matter  is  even  less  difficult.  Nearly  all  varieties  of  fruits, 
flowers  and  vegetables  propagated  in  this  way  are  notori- 
ously unstable  when  grown  from  seed.  The  excellent 
varieties  that  we  now  have  midoubtedly  owe  their  supe- 
riority in  large  measure  to  a  fortunate  combination  of 
manv  different  characters  so  made  as  to  obtain  the  maxi- 
mum  effect  from  hybrid  vigor.  Attempting  to  obtain 
further  improvement  by  crossing  these  already  widely 
crossed  varieties  is  like  trying  to  solve  a  picture  puzzle 
in  the  dark.  First  analyze  the  material  to  be  used  by 
systematic  and  rigorous  inbreeding,  let  the  consequences 
be  what  they  may.  Then  cross  the  different  constant 
types  which  may  be  ultimately  obtained  and  test  one  com- 
bination after  another  until  a  real  improvement  is  effected. 
When  that  is  done  the  individuals  can  be  propagated  in- 
definitely by  the  same  means  utilized  before.  Of  course, 
this  method  has  the  objection  that  many  of  the  plants 
propagated  asexually  require  several  years  for  each 
sexual  generation.  Results  would  be  slow  for  that  reason, 
it  is  true,  but  they  would  be  sure. 

With  animals  the  application  of  this  method  would  be 
quite  a  different  proposition.  Inbreeding  closer  than 
brother  and  sister  mating  could  not  be  practiced,  and  the 
time  required  to  obtain  purity  and  constancy  would  be 
much  greater  than  is  the  case  with  self-fertilization. 
Moreover,  the  number  of  individuals  which  could  be  ob- 
tained would  be  so  small  that  selection  could  not  be  made 
advantageously.  Finally,  the  cost  of  raising  most  ani- 
mals is  so  great  that  the  maintenance  of  animals  of  little 


PLANT  AND  ANIMAL  IMPROVEMENT        219 

or  no  value  in  themselves  solely  for  a  possible  ultimate 
improvement  miglit  well  be  too  discouraging  an  under- 
taking. But  what  could  be  done  is  to  use  animals  from 
some  of  the  intensively  inbred  herds  of  the  present  day 
as  basic  stock  for  building  up  new  strains  through  cross- 
breeding and  selection.  The  point  which  we  particularly 
wish  to  make  here  is  that  the  apparently  disastrous 
effects  of  inbreeding  need  not  be  so  greatly  feared  as  is 
usually  the  case ;  because  if  anything  is  lost  by  inbreeding 
it  is  usually  something  undesirable.  Inbreeding,  there- 
fore, may  prove  to  be  a  very  great  gain  if  used  as  a 
method  of  purifying  and  analyzing  a  cross-bred  stock. 

Wliile  the  full  value  of  inbreeding  in  plant  and  animal 
improvement  has  not  as  yet  been  fully  recognized,  the 
advantages  derived  from  outbreeding  are  more  generally 
known.  Outbreeding  as  a  means  of  improvement  may  be 
considered  under  two  heads :  First,  the  immediate  value  to 
be  derived  from  crossing  related  types  and  thus  securing 
the  maximum  benefit  from  hybrid  vigor ;  second,  the  more 
complex  problem  of  crossing  radically  different  forms  to 
create  variability  out  of  which  new  breeds  or  new  varie- 
ties may  be  constructed  by  a  process  of  selection. 

In  some  cases  the  first  generation  cross,  although 
vigorous,  is  sterile.  An  example  is  the  mule,  which, 
though  having  the  disadvantage  of  not  being  able  to  re- 
produce, has  held  a  place  in  agriculture  and  industry 
throughout  historic  times.  According  to  Mumford  ^^'^ 
there  were  nearly  five  millions  of  these  animals  in  the 
United  States  in  1915.    Of  it  he  says : 

This  was  more  than  one-fifth  of  the  total  number  of  horses  in  the 
comitry  at  the  time.  The  |)roduotion  of  mules  has  increased  at  a  more 
rapid  rate  than  hoi-ses,  and  the  use  of  mules  is  becoming  more  exten- 


220         INBEEEDING  AND  OUTBREEDING 

sive.  The  mule  hybrid  is  a  remarkable  example  of  the  practical  ad- 
vantagfes  which  follow  a  particular  cross.  This  animal  is  more  hardy 
and  enduring  than  either  parent.  As  compared  with  the  horse,  the  mule 
is  longer  lived,  less  subject  to  disease  or  injury,  and  more  efficient  in 
the  use  of  food.  The  mule  can  be  safely  put  to  work  at  a  younger 
age,  will  thrive  on  coarser  feed,  and  seems  to  be  much  better  able  to 
avoid  many  dangere  which  menace  the  usefulness  of  the  horse.  The 
mule  will  perform  more  arduous  labor  on  less  food.  The  mule  will 
endure  the  heat  of  southern  latitudes  more  successfully  than  the  horse 
and  is  therefore  a  more  popular  draft  animal  in  the  South. 

Other  first  generation  crosses  among  animals,  which 
are  not  sterile  like  the  mule,  have  good  qualities  and  are 
well  known.  Youatt,  early  in  the  nineteenth  centuiy, 
stated  that  crosses  between  the  English  and  Chinese 
breeds  of  s\vine  were  frequently  made,  and  that  in  Ger- 
many the  native  breeds  were  often  crossed  with  the  Eng- 
lish breeds.  To-day  the  first  generation  cross  between  the 
Duroc-Jersey  and  the  Poland-China,  and  between  the 
Poland-China  and  Chester  White  are  popular  animals 
among  the  feeders.  No  attempt  is  made  to  breed  from 
them  as  it  is  well  known  that  the  later  generations  are 
variable  in  color,  size  and  conformation,  and  usually 
possess  less  vigor  than  the  animals  of  the  original  cross. 

First  generation  crosses  between  many  of  the  stand- 
ard breeds  of  beef  cattle  are  raised,  and  frequently  they 
whi  the  first  prizes  at  the  stock  shows.  The  Shorthorn 
and  Aberdeen-Angus  combination  is  popular. 

The  Mediterranean  breeds  of  poultry  are  sometimes 
crossed  with  the  heavier  types.  First  crosses  of  Leg- 
horns and  Plymouth  Eocks  give  birds  which  are  not  so  apt 
to  become  over-fat  and  yet  are  more  valuable  for  meat 
than  the  smaller  Leghorns. 

The    opi>ortunities    for    improvement    in    this    way 


Fig.  45. — "Big  Jim,"  the  product  uf  a  pure  hvcd  Percheron  stallion  mated  witli  a 
grade  mare  of  the  same  breed,  showing  the  value  of  coneentrating  desirable  (lualities  liy 
close  breeding  in  pure  bred  livestock.  (From  Sanders  and  Dinsmore  in  "A  History  of 
the  Percheron  Horse.") 


PLANT  AND  ANIMAL  IMPROVEMENT       221 

through  the  utilization  of  hybrid  vigor  are  no  less  great 
in  plants.  The  increased  cost  of  seed  is  an  item  and  the 
practice  can  only  be  followed  with  those  plants  which  are 
easily  crossed  and  which  produce  a  large  amount  of  seed. 
Many  plants  in  which  production  might  be  increased  in 
this  way  have  such  low  economic  value,  however,  that  it 
would  not  be  profitable  to  utilize  the  method.  Cases  in 
point  are  squashes  and  pumpkins.  Tomatoes  and  cucum- 
bers in  certain  crosses,  on  the  other  hand,  have  been 
found  to  give  appreciable  increases  in  yield  and  other 
desirable  qualities,  advantages  which  are  readily  secured 
every  time  the  particular  cross  is  made. 

Maize  is  the  plant  which  is  most  suitable  for  use  in  this 
way,  a  notable  fact  since  it  is  the  most  valuable  farm 
crop  in  the  Western  Hemisphere.  The  reason  it  merits 
this  statement  is  because  it  is  easily  crossed  on  a  large 
scale  by  sowing  the  two  types  to  be  crossed  in  alternate 
rows  in  an  isolated  plot  and  detasseling  all  of  one  kind 
before  pollen  is  shed.  As  early  as  1876  Beal  ^  reported 
that  com  could  be  increased  in  yield  in  this  way.  Since 
that  time  numerous  tests  have  been  made  and  the  fact  is 
established  that  crosses  between  varieties  of  com  of  some- 
what different  type  may  be  expected  to  outyield  either 
parent  in  many  cases,  and  when  the  parental  varieties 
differ  in  time  of  maturing  may  be  expected  to  ripen 
earlier  than  the  later  parent.  Thus  out  of  fifty  first  gen- 
eration crosses  between  varieties  of  com  grown  in  Con- 
necticut, eighty-eight  per  cent,  yielded  more  than  the 
average,  and  sixty-six  per  cent,  yielded  more  than  either 
parent.  The  average  increase  in  all  the  crosses  above  the 
average  of  their  parents  was  about  ten  per  cent.,  includ- 
ing the  crosses  which  gave  no  indication  of  hybrid  vigor. 


222         INBREEDING  AND  OUTBEEEDING 

The  greatest  increases  occurred  in  the  crosses  between 
flint  and  dent  varieties,  and  often  there  was  a  really  note- 
worthy hastening  of  the  time  of  ripening,  which  is  of  con- 
siderable importance  in  those  regions  where  early  fall 
frosts  are  a  limiting  factor. 

The  greatest  improvement  to  be  made  in  this  way 
comes  from  crossing  varieties  which  have  previously  been 
put  through  a  process  of  self-pollination.  When  certain 
inbred  strains  are  crossed  the  increase  in  growth  is  re- 
markable, as  previously  noted.  This  comes  partly  from 
the  fact  that  following  inbreeding  the  maximum  effect  of 
hybrid  vigor  is  obtained  while  in  ordinary  varieties  seg- 
regation brings  about  partial  homozygosity  in  many 
plants.  It  is  also  due  to  the  elimination  of  many  unde- 
sirable characters  during  the  process  of  inbreeding.  The 
crossed  plants  are  remarkably  uniform.  One  plant  is  a 
replica  of  another.  Given  proper  conditions  the^^  all  pro- 
duce good  ears  which  form  a  remarkable  contrast  to  ordi- 
nary varieties  in  their  similarity  to  each  other.  There 
are  no  barren  stalks,  and  the  abnormalities  and  mon- 
strosities which  commonly  occur  in  every  field  of  corn  are 
almost  entirely  absent.  In  those  cases  in  which  one  or 
both  of  the  parent  strains  is  resistant  to  parasitic  infec- 
tion, such  as  smut,  the  cross  is  also  resistant  and  this  is  a 
factor  for  greater  r)roduction. 

There  are,  on  the  other  hand,  certain  serious  disad- 
vantages in  the  practical  utilization  of  first  generation 
crosses  between  inbred  strains.  In  the  first  place  the 
yields  of  the  inbred  plants  are  low,  which  makes  the  cost 
of  the  crossed  seed  high.  What  is  more  serious,  the  seeds 
produced  on  inbred  plants  are  small  and  less  well  devel- 
oped than  seeds  of  ordinary  corn,  and  the  seedlings  com- 


PLANT  AND  ANIMAL  IMPROVEMENT        223 


ing  from  these  seeds  are  less  vigorous  and  are  thereby 
greatly  handicapped  at  the  start.  The  plants  at  first  are 
smaller  and  have  a  less  healthy  color  than  plants  of  ordi- 
nary varieties,  and  although  they  usually  overcome  this 
handicap,  they  may  not  always  do  so  if  the  conditions  in 
the  earlier  part  of  the  season  are  particularly  unfavorable. 
A  method  which  overcomes  these  objections  is  now 
being  tested  at  the  Connecticut  Agricultural  Experiment 
Station,  and  promises  excellent  results.  This  method  is 
as  follows :  Four  inbred  strains  are  selected  which  when 
tested  by  crossing  in  all  the  six  different  combinations 
give  an  increased  yield.  Two  of  these  strains  are  crossed 
to  make  one  first  generation  hybrid  and  the  other  two  are 
crossed  to  give  another.  These  two  different  crosses, 
which  are  large  vigorous  plants,  are  again  crossed  and  the 
seed  obtained  used  for  general  field  planting.  This  pro- 
cedure may  be  diagrammed  as  follows: 

Original  variety 


Inbred  strains 


First  generation  crosses 


(AXB) 


(CXI)) 


Double  first  generation  cross 


{AXB)XiCXD) 


In  this  way  large  yields  of  well-developed  seed  are 
obtained,  and  the  young  plants  are  not  handicapped  in  any 
way.    The  beautiful  uniformity  of  the  first  cross  is  sacri- 


224         INBEEEDING  AND  OUTBREEDING 

ficed,  but  the  advantages  gained  promise  to  counterbal- 
ance any  loss  in  this  respect.  Theoretically  there  is  little 
reduction  in  heterozygosity  and  presumably  little  reduc- 
tion in  the  incentive  towards  increased  size  and  produc- 
tiveness. A  great  many  different  possibilities  are 
involved  in  such  double  crossing  and  they  have  not  been 
sufficiently  tested  to  warrant  extravagant  claims,  but 
judging  by  their  appearance  such  doubly-crossed  plants 
are  clearly  the  finest  specimens  of  corn  so  far  obtained 
under  the  conditions  in  which  they  have  been  tested. 

The  first  impression  probably  gained  from  the  outline 
of  this  method  of  crossing  corn  is  that  it  is  a  rather  com- 
plex proposition.  It  is  somewhat  involved,  but  it  is  more 
simple  than  it  seems  at  first  sight.  It  is  not  a  method  that 
will  interest  most  farmers,  but  it  is  something  that  may 
easily  be  taken  up  by  seedsmen;  in  fact,  it  is  the  first  time 
in  agricultural  history  that  a  seedsman  is  enabled  to  gain 
the  full  benefit  from  a  desirable  origination  of  his  own  or 
something  that  he  has  purchased.  The  man  who  origi- 
nates devices  to  open  our  boxes  of  shoe  polish  or  to 
autograph  our  camera  negatives,  is  able  to  patent  his 
product  and  gain  the  full  reward  for  his  inventiveness. 
The  man  who  originates  a  new  plant  which  may  be  of 
incalculable  benefit  to  the  whole  country  gets  nothing — 
not  even  fame — for  his  pains,  as  the  plants  can  be  propa- 
gated by  anyone.  There  is  correspondingly  less  incentive 
for  the  production  of  improved  types.  The  utilization  of 
first  generation  hybrids  enables  the  originator  to  keep  the 
parental  types  and  give  out  only  the  crossed  seeds,  which 
are  less  valuable  for  continued  propagation. 

The  second  phase  of  the  subject  of  outbreeding  in  its 
relation  to  plant  and  animal  improvement — that  of  wide 


Fic.  4G. — First  generation  cross  of  Chester  Wliite   and  Poland  China.     (From  Detlefsen.1 


PLANT  AND  ANIMAL  IMPEOVEMENT  225 

crossing  between  distinct  varieties,  species  or  even  genera 
— is  so  large  a  topic  it  cannot  be  more  than  touched  upon 
here.  Each  particular  cross  presents  technical  problems 
of  its  own.  All  one  can  say  as  a  generality  is  that  the 
principle  in  every  case  is  the  same.  Crossing  brings  to- 
gether germ  plasms  having  various  attributes.  These 
attributes,  the  hereditary  factors,  recombine  with  regu- 
larity and  precision.  They  Mendelize.  Prom  Mendelian 
segregation  and  recombination  come  the  possibilities  of 
new  and  improved  races.  Except  in  those  rare  instances 
when  new  variations  previously  unknown  to  the  species 
occur,  nothing  can  come  out  of  the  cross  that  did  not  go 
in.  But  the  number  of  combinations  possible  when  the 
two  parents  differ  by  many  hereditary  factors  is  so  great 
that  practically  speaking  many  character  complexes  may 
appear  which  have  never  before  had  the  chance  of  showing 
their  merits  or  defects.    In  them  lie  our  hopes. 

It  was  noted  earlier  that  many  species  crosses  are  par- 
tially sterile,  that  there  is  often  a  degeneration  of  many 
of  the  germ  cells  and  embryos,  and  that  certain  extreme 
types  are  thereby  produced  more  frequently  than  is  usu- 
ally to  be  expected.  The  extreme  variability  induced  by 
such  wide  crossing  offers  the  best  field  in  which  to  look  for 
the  beginnings  of  new  and  valuable  types  of  animals  and 
plants.  This  is  not  a  theory;  it  is  a  general  fact  born  of 
long  experience,  for  when  we  look  into  the  origin  of  many 
of  our  most  valuable  domesticated  animals  and  plants  we 
find  unmistakable  evidence  of  their  hybrid  ancestry. 


15 


CHAPTER  XII 

INBEEEDING    AND     OUTBREEDINQ    IN    MAN: 
THEIR  EFFECT  ON  THE  INDIVIDUAL 

The  world  has  entered  an  age  of  reason.  The  leaven 
of  education  is  working  rapidly,  and  all  relations  of  man 
to  his  fellow-man,  all  connections  of  man  with  his  environ- 
ment, are  being  subjected  to  thorough  scrutiny.  To  ac- 
company the  current  changes  in  the  arts  due  to  new 
advances  in  science,  enlightened  democracy  demands 
progress  in  religion  and  philosophy,  in  government  and 
social  policy.  It  has  set  upon  its  program  the  task  of 
establishing  a  broad  scheme  of  social  hygiene,  and  more 
than  one  might  suspect  has  been  accomplished. 

Although  there  is  still  room  for  improvement,  general 
communistic  sanitation  has  reached  a  degree  of  efficiency 
which  a  few  years  ago  would  hardly  have  been  deemed 
possible.  The  civilized  world  has  gone  through  a  clean- 
ing-up  period  which  has  provided  reasonably  hygienic 
buildings,  tidy  streets  and  excellent  waste  disposal ;  Avhich 
has  bettered  the  condition  of  the  people  and  lowered  their 
death  rate  by  quarantine  regulations,  public  hospitals, 
and  free  medical  attention  in  the  schools ;  and  has  passed 
on  to  preventive  work,  vaccination,  pure  food  legislation, 
and  the  like.  There  has  been  marked  progress  in 
ameliorating  conditions  of  work.  Sanitation  has  been 
made  the  subject  of  many  laws ;  hours  of  toil  shortened — 
particularly  for  women  and  children.  Factory  super- 
vision and  wage  regulation  are  accepted  facts ;  industrial 
insurance  is  in  the  air.    Public  education  has  made  strides 

226 


MAN  227 

with  seven  league  boots.  That  dignilied  monument,  the 
free  school,  is  not  the  only  evidence.  There  are  normal 
schools  and  universities,  museums  and  research  institu- 
tions, public  collections  of  books  and  public  printing 
of  books  in  numbers  sufficient  to  form  libraries 
by  themselves. 

Is  it  realized  just  what  this  means — why  social  policy 
has  developed  in  precisely  this  manner?  It  is  because 
this  is  the  mental  line  of  least  resistance,  the  order  of 
social  reform  needing  the  least  foresight.  The  first  efforts 
were  to  clear  up  obvious  filth,  the  accumulated  debris  of 
human  activity — the  record  of  the  past ;  the  step  f oi^^ard 
was  an  appreciation  of  the  efficiency  in  production  result- 
ing from  comfort  and  satisfaction  in  conditions  of  work — 
the  present;  and  then  came  the  spread  of  educational 
facilities — a  preparation  for  work,  an  insurance  on  the 
immediate  future. 

But  change,  progress,  reform,  whatever  one  may  call 
it,  ought  not  and  will  not  stop  here.  The  program  of  social 
hygiene  is  not  complete  if  there  is  failure  to  provide  for  a 
future  still  more  distant.  And  this  is  the  real  thought  in 
the  minds  of  a  few  clear  thinkers  of  Europe  and  America 
whose  names  are  connected  with  the  spread  of  eugenic 
policies.  It  was  thought  for  the  care  of  the  coming  gen- 
eration that  led  Budin  to  establish  the  Infant  Consulta- 
tions and  MilJc  Depots  in  Paris,  that  led  Miele  to  start  his 
School  for  Mothers  in  Ghent.  It  was  thought  for  the 
future  of  the  race  as  a  whole  that  gave  the  impulse  to 
Galton's  work. 

We  have  no  eugenic  system  of  conduct  to  lay  down 
here,  for  we  believe  the  acquisition  and  diffusion  of  knowl- 
edge are  needed  more  than  widespread  dogma  and  ill- 


228         INBREEDING  AND  OUTBREEDING 

advised  legislation  at  the  present  day.  The  recommenda- 
tions of  the  sympathetic  altruist  with  a  little  learning  have 
done  more  than  anything  else  to  hinder  a  healthy  growth 
of  eugenic  ideas.  All  we  would  ask  is  that  the  physician, 
the  clergyman,  the  social  worker,  the  penologist,  the 
statesman,  give  conscientious  consideration  to  the  facts 
of  heredity  as  a  guiding  principle  in  the  solution  of  the 
problems  of  the  family  with  which  they  have  to  do.  No 
questions  are  so  hedged  about  with  superstition,  with 
irrational  tradition,  with  religious  dogma,  as  those  which 
concern  sex  and  reproduction;  no  problems  are  more 
delicate,  more  difficult,  than  those  which  seek  the  direction 
of  human  evolution ;  yet  after  all  man  is  an  animal  and 
must  be  dealt  with  as  such.  Civic  law  he  may  escape,  to 
natural  law  there  is  no  immunity. 

We  have  seen  how  characters  are  transmitted  in 
sexual  reproduction  in  the  lower  animals  and  in  plants, 
how  hereditary  differences  carried  as  potentialities  in  the 
germ  cells  ^re  shuffled  and  divided  when  these  are  formed, 
by  a  law  as  definite  and  precise  as  one  of  chemistry  or 
physics.  "We  have  seen  how  the  operation  of  this  law 
brings  about  the  outstanding  phenomena  of  inbreeding 
and  outbreeding.  Man  is  just  another  sexually  reproduc- 
ing mammal  and  a  priori  his  heredity  is  guided  by  this 
law.  Being  a  thoroughgoing  egotist,  he  doesn^t  like  to 
realize  this.  It  takes  time  for  the  truth  to  filter  in.  Com- 
parative anatomy  and  physiology  and  the  doctrine  of  evo- 
lution have  been  the  greatest  agents  in  this  familiariza- 
tion process.  The  veriest  schoolboy  now  recognizes  the 
homologies  between  the  bones  and  muscles  of  the  lower 
mammals  and  those  of  man,  and  sees  nothing  out  of  the 
ordinary  that  their  digestive  metabolisms  are  the  same. 


MAN  229 

Those  with  no  biological  training  have  now  no  difficulty 
in  accepting  as  fact  the  idea  that  man  came  into  being  by 
the  same  process  of  evolution  as  the  rest  of  the  organic 
world.  But  even  in  these  cases  it  has  been  a  long  struggle 
against  prejudice,  and  the  scientific  study  of  heredity  is 
too  recent  to  have  outgrown  it.  We  will,  therefore,  not 
confine  our  argument  strictly  to  the  logic  of  the  question. 
Inheritance  in  man  has  actually  been  studied  by  the  same 
general  methods  as  have  brought  such  wonderful  results 
in  other  organisms,  and  corroboration  of  every  detail  has 
been  the  outcome. 

When  one  says  the  fundamentals  of  Mendelism  have 
been  supported  in  detail  by  investigations  on  the  human 
race,  he  does  not  mean  to  unply  that  the  critical  investiga- 
tions needed  to  establish  the  Mendelian  hypothesis  in  the 
beginning  were  supplied  by  such  data.  This  is  obviously 
impossible  because  of  inability  to  control  matings.  The 
only  records  which  can  be  analyzed  are  pedigrees  of 
families  carrying  some  striking  hereditary  phenomenon. 
Such  a  method  is  unsatisfactory  because  the  data  must  be 
gathered  second-hand  through  several  generations — 
often  by  untrained  workers.  It  is  necessary  to  work 
backward  instead  of  forward,  to  be  content  with  frag- 
mentary information,  to  realize  the  high  percentage  of  ex- 
perimental error.  What  is  meant  by  corroboration  of 
Mendelism  in  human  heredity  is  simply  that  starting  with 
the  assumption  of  the  truth  of  the  law,  all  human  data 
have  been  found  to  fit.  But  it  is  often  very  difficult  to  say 
whether  the  inheritance  of  a  particular  human  trait  is 
dominant  or  recessive,  whether  it  is  controlled  by  one  or 
by  several  factors,  whether  it  is  sex-linked  or  independent. 

Several  skeletal  abnormalities  unquestionably  show  a 


230         INBEEEDING  AND  OUTBREEDING 

high  degree  of  dominance.  Among  them  ma,j  be  men- 
tioned the  peculiar  type  of  dwarfing  known  as  achondro- 
plasty,  and  the  various  digital  malformations  termed 
medically  brachydactyly,  Polydactyly  and  syndactyly. 
Evidence  of  complete  dominance  is  probably  better  in 
these  than  in  any  other  cases,  but  in  view  of  the  many 
instances  where  subsidiary  factors  enhance  or  diminish 
the  expression  of  a  primary  factor,  it  seems  decidedly 
unwise  to  follow  Davenport  and  to  recommend  marriage 
with  unaffected  members  of  such  families  with  the  assur- 
ance that  the  latter  cannot  transmit  the  trouble  which 
afflicts  their  relatives.  If  this  advice  could  be  accepted  in 
good  faith,  the  inheritance  of  dominant  traits,  whether 
disagreeable  or  desirable,  would  have  little  interest.  They 
would  stand  revealed  in  those  possessing  them;  they  alone 
could  transmit  them.  But  this  is  not  the  whole  truth. 
Some  of  the  so-called  dominant  characters  in  man  are  ab- 
normalities which  no  one  cares  to  see  expressed  in  his  or 
her  children,  and  their  dominance  is  imperfect  or  uncer- 
tain. In  many  instances  the  records  have  been  analyzed 
hastily  and  carelessly;  for  example,  hare-lip  and  cleft 
palate,  which  is  clearly  a  recessive  condition  in  face  of 
the  data,  though  passing  as  dominant  in  the  various  text- 
books of  heredity.  In  all  cases  there  is  no  guarantee  that 
the  unaffected  member  of  the  stricken  family  is  germinally 
a  pure  normal.  The  family  is  one  whose  alliance  is  not  to 
be  sought  by  those  who  have  a  proper  pride  in  a  normal 
healthy  posterity.  Let  us  enumerate  some  of  the  troubles 
that  come  in  this  category:  Hereditary  cataract,  ichy- 
thyosis  or  scaly  skin,  defective  hair  and  teeth,  diabetes 
insipidus,  Huntington  ^s  chorea — an  affection  of  the 
nervous    system,    imperfectly    developed    sex    organs. 


MAN  231 

Does  any  one  desire  the  establishment  of  sub-races 
thus  characterized? 

Other  undesirable  traits  are  more  certainly  recessive 
and  the  heterozygous  carriers  of  the  factors  which  control 
them  cannot  be  distinguished  by  any  differentiating  char- 
acters of  their  own.  Some  of  these  abnormalities  are 
extremely  rare  and  for  various  reasons  are  not  likely  to 
increase.  Among  them  may  be  mentioned  pigmentary  de- 
generation of  the  retina,  Friedrich's  ataxia,  and  xero- 
derma pigTuentosum.  But  there  are  others  which  well  may 
give  some  cause  for  dismal  forebodings — hereditary 
feeble-mindedness  and  some  forms  of  epilepsy  and  in- 
sanity. These  characters  may  be  put  do^vn  as  largely 
hereditary,  and  probably  transmitted  as  single  Mendelian 
units,  but  it  must  not  be  supposed  that  each  manifestation 
of  them  is  of  similar  kind.  From  the  graduated  character 
of  feeble-mindedness  and  from  the  frequency  with  which 
epilepsy  and  other  forms  of  neurosis  appear  in  feeble- 
minded families,  it  is  reasonable  to  suppose  that  minor 
factors  of  several  types  play  a  part.  Nevertheless,  for  the 
deductions  we  wish  to  make  here,  they  may  be  accepted  as 
true  examples  of  Mendelian  recessiveness. 

Other  characters  are  not  so  simple  in  their  inheritance. 
The  Davenports^^'^^'^^'^^  have  collected  a  large  amount 
of  data  on  the  inheritance  of  skin  color  in  negro-white 
crosses,  the  inheritance  of  hair  color  in  Caucasian  mix- 
tures, and  the  inheritance  of  normal  differences  in  stature. 
These  characters  are  all  complex.  They  are  transmitted 
just  as  are  the  differences  in  height  in  plants — more  or 
less  of  a  blend  in  the  first  hybrid  generation,  and  the 
appearance  of  such  second  generation  types  as  would  be 
expected  if  the  differences  were  controlled  by  the  segre- 


232         INBEEEDING  AND  OUTBKEEDING 

gation  and  recombination  of  several  factor  pairs.  This  in 
general  is  the  interpretation  given  the  inheritance  of  gen- 
eral mental  ability  or  inherent  ability  in  music,  literature, 
art,  or  mathematics.  We  simply  know  that  such  abilities 
are  inherited  in  some  complex  way,  which,  it  is  logical  to 
assmne,  is  Mendelian.  We  know  the  fact  from  pedigrees 
of  families  in  which  ability  of  a  particular  kind  is  very 
marked;  we  make  the  assumption  from  such  circumstan- 
tial evidence  of  the  generality  of  Mendelian  phenomena 
as  has  been  presented  in  abstract  in  this  volume. 

Having  this  basis,  what  shall  be  said  of  the  effect  of 
inbreeding  and  crossing  on  the  individual!  It  would  be 
easy  to  point  to  the  conclusions  reached  when  discussing 
domestic  animals  and  plants,  and  say:  **The  same  line  of 
reasoning  holds  for  man;  draw  your  own  conclusions. ' ' 
But  this  is  hardly  satisfactory.  It  is  true  enough,  as  a 
generality,  to  point  to  the  desirability  of  some  mating  out- 
side a  particular  line  in  order  to  assure  physical  vigor  by 
complementary  hereditary  factors  meeting  each  other,  or 
to  mention  the  possibility  of  undesirable  characters  being 
brought  to  light  in  some  strains  and  of  desirable  char- 
acters being  added  in  others  by  inbreeding.  One  would 
hardly  feel  this  to  be  an  answer  to  the  question.  If 
the  study  of  heredity  has  resulted  in  an  advance  in  knowl- 
edge having  some  practical  value,  it  ought  to  be  possible 
to  make  a  more  definite  analysis  of  the  facts  as  applied  to 
the  human  race. 

Let  us  ask  first,  What  is  ability  in  the  human  race,  and 
what  the  evidence  that  it  is  inherited  ?  A  fair  definition  of 
ability  may  be  given  in  the  phrase,  *  *  skill  in  accomplish- 
ment, ' '  and  this  puts  considerable  emphasis  on  mentality. 
We  all  desire  a  healthy  mind  in  a  healthy  body,  but  a 


MAN  233 

feeble-minded  Goliath  is  hardly  of  much  use  in  the  world, 
while  a  Eobert  Louis  Stevenson  struggling  through  life 
with  the  handicap  of  a  delicate  constitution  leaves  an  im- 
perishable monument.  At  any  rate,  there  are  few  who 
deny  the  inheritance  of  physical  differences.  Pedigrees 
showing  the  exact  method  of  inheritance  of  physical  traits 
are  too  numerous. 

The  first  real  study  of  the  inheritance  of  mental 
capacity  was  Galton's  *' Hereditary  Genius,''  jDublished  in 
1869.'^^  By  comparing  the  attainments  of  the  relatives  of 
eminent  men  from  the  United  Kingdom  with  the  attain- 
ments of  its  population  as  a  whole  he  proved  beyond  a 
reasonable  doubt  the  inheritance  of  potential  capacity, 
though  he  had  no  inkling  of  how  this  capacity  was  trans- 
mitted. His  conclusions  have  been  corroborated  by  the 
works  of  Havelock  Ellis ®^  on  ^^British  Men  of  Genius"; 
of  Woods  22^  on  **  Heredity  in  Eoyalty,"  where  lack  of 
opportunity  did  not  play  such  a  disturbing  role ;  and  of 
Cattell,^^  Nearing,  ^^^'  ^^^  and  Davenport**'  on  eminent 
Americans.  Perhaps  the  most  striking  feature  of  Gal- 
ton's researches  is  the  evidence  of  rarity  of  genius  among 
a  people  who  have  contributed  the  greatest  amount  of 
creative  work  of  the  first  magnitude  in  modern  times  (see 
Merz  ^'*^).  Only  two  hundred  and  fifty  men  per  million 
of  the  British  population  became  eminent.  Though  un- 
questionably this  proportion  must  be  increased  several 
times  because  of  the  lack  of  opportunity  of  those  similarly 
endowed  to  give  full  rein  to  their  capacity,  and  because 
eminence  as  measured  by  history  is  fallacious  in  the  ex- 
treme, nevertheless,  when  translated  into  the  terms  of 
modem  genetics  this  ratio  has  a  definite  meaning.  The 
hereditary  factors  which  contribute  toward  the  possibility 


234         INBREEDING  AND  OUTBREEDING- 

of  genius  are  numerous.  Only  occasionally  is  the  proper 
combination  brought  together.  The  factors  exist  in  the 
population  at  large,  distributed  in  part  to  one  individual, 
in  part  to  another,  but  in  the  main  the  combinations  make 
but  for  mediocrity.  Only  on  a  rare  occasion  is  a  favored 
one  so  showered  with  these  gifts  that  he  stands  out 
supreme  among  his  fellow-men. 

No  one  knows  what  the  component  parts  of  these  desir- 
able qualities  are,  or  can  distinguish  by  external  traits  the 
individual  who  carries  them,  but  a  knowledge  of  the  opera- 
tion of  Mendelian  heredity  enables  one  to  say  in  a  general 
way  what  ought  to  occur  under  given  conditions  with  the 
same  confidence  as  when  dealing  with  similar  indefinite 
qualities  in  the  lower  animals.  Close  selection,  inbreeding, 
tends  toward  the  production  of  gametic  purity  with  mathe- 
matical precision.  Does  any  one  doubt  but  that  close 
breeding  in  families  which  have  shown  superior  civic  value 
tends  to  concentrate,  to  purif}^,  in  genetic  terms  to  render 
homozygous,  the  particular  factorial  combinations  which 
cause  this  superior  endowment!  Will  any  one  deny  there 
is  a  real  privilege  in  being  allowed  to  marry  into  a  family 
of  proved  worth,  or  a  real  reason  for  that  family  to  scru- 
tinize carefully  the  ancestry  of  one  who  asks  to  become 
allied  with  it? 

We  have  seen  how  when  certain  hereditary  factors 
have  been  brought  together  by  the  proper  breaks  in  link- 
age, they  tend  to  be  transmitted  together  in  the  same  man- 
ner as  did  the  previous  set  of  coupled  factors.  This 
same  idea,  followed  to  its  logical  conclusion,  is  a  great 
help  in  visualizing  the  inheritance  of  capacity.  The  indi- 
vidual who  owes  his  capacity  to  a  complex  which  mav  be 
represented  gi^aphically  by  the  linked  series  AhCd-aBcD 


MAN  235 

where  the  capital  letters  represent  the  desirable  factors, 
must  be  much  more  common  than  the  individual  who  by 
linkage  breaks  receives  the  inheritance  ABC D -abed; 
yet  the  latter  is  the  one  who  has  the  greatest  power  of 
transmitting  his  endowments.  Of  the  influence  of  the 
individual  in  heredity,  much  has  been  w^ritten,  particu- 
larly by  those  who,  great  themselves,  have  founded  great 
families  in  American  history.  Elizabeth  Tuttle  through 
Jonathan  Edwards,  William  Fitzhugh,  the  three  Lees — 
sons  of  Eiehard  Lee,  and  the  various  lines  established  by 
John  Preston — Venable,  Payne,  Wooley  and  Breckin- 
ridge— are'  examples.  It  seems  most  reasonable  to  sup- 
pose that  such  prepotency  for  good  comes  about  by  the 
gathering  together  of  groups  of  significant  factors  in  the 
manner  outlined  above.  Can  one  object  to  the  concentra- 
tion of  such  worth  by  relatively  strict  inbreeding  despite 
its  possibilities  for  ill!  In  fact,  if  we  examine  carefully 
the  geneological  records  of  such  families,  marriage  of 
near  relatives  is  found  to  be  a  common  occurrence.  AVould 
it  not  be  wise  to  do  away  with  statutes  against  the  mar- 
riage of  first  cousins  such  as  are  laid  doAvn  in  the  laws  of 
nearly  half  our  States,  even  though  the  argument  on  the 
other  side,  as  we  shall  show,  is  just  as  great  ?  If  such  laws 
had  been  followed  in  every  mating  the  world  would  have 
lost  an  Abraham  Lincoln  and  have  been  compelled  to 
punish  a  Charles  Darwin. 

The  mention  of  the  name  of  the  great  Civil  War  Presi- 
dent doubtless  brings  the  question :  How  does  one  account 
for  his  capacity  on  this  hypothesis?  The  question  is  well 
placed.  Such  geniuses  as  Lincoln,  Dalton,  Faraday, 
Franklin,  Pasteur — scions  of  the  coimiion  people,  simple 
examples  of  greatness  standing  among  the  commonplace — 


236         INBREEDING  AND  OUTBREEDING 

have  been  the  stock  arguments  of  those  sociologists  who 
beheve  every  man  a  star  of  the  hrst  magnitude  darkened 
by  lack  of  opportunity.  Let  us  consider  such  cases  in 
some  detail. 

The  worst  trouble  with  the  euthenic  idealists  is  that 
superhcially  they  are  not  so  far  wrong,  although  funda- 
mentally they  miss  the  point  entirely.  Greatness  in  this 
world  is  governed  by  many  factors.  Appreciating  Lincoln 
and  Franklin  for  all  they  were,  it  is  still  allowable  to 
question  whether  there  were  not  some  thousands  of  others 
of  the  same  periods  who  would  have  entered  the  Hall  of 
Fame  had  they  been  given  the  same  political  environ- 
ments. Many  of  these  contemporaries  doubtless  were 
great  in  the  callings  to  which  they  were  turned  by  force  of 
circumstances ;  but  the  world  seldom  makes  a  very  wide 
path  to  the  door  of  him  who  makes  a  better  mouse-trap 
than  his  neighbor.  Castle  ^^  has  called  attention  to  the 
notable  astronomers,  the  eminent  biologists,  inspired  by 
the  teachings  of  Briinnow  and  of  Agassiz.  If  Briinnow 
had  found  a  home  at  Princeton  instead  of  Michigan,  and 
Agassiz  a  place  at  Yale  instead  of  Harvard,  new  names 
would  be  found  among  American  astronomers  and  biolo- 
gists, but  their  number  would  not  be  less.  In  short,  one 
may  admit  with  the  euthenist  the  role  of  chance,  of  oppor- 
tunity, of  personal  influence,  of  political  preferment,  of 
economic  stress,  in  the  moulding  of  men;  one  may  ac- 
knowledge the  difficulties  attending  the  ranking  of  the 
great  and  the  near-great;  and  yet  abate  not  a  jot  or  tittle 
of  the  position  that  inherent  capacity,  inherited  potenti- 
ality, lies  at  the  base  of  all.  It  is  the  one  solid  foundation 
on  which  to  build. 

Could  one  gauge  the  ability  of  the  progenitors  of 


MAN  237 

Franklin  and  Pasteur  in  some  other  way  than  by  the  bio- 
graphical dictionary,  they  would  probably  be  found  to 
have  a  fair  share  of  natural  gifts.  Their  family  lines  are 
not  to  be  compared  with  those  of  the  notorious  Zeros, 
Jukes  and  Nams,  where  the  individuals  through  their  bad 
heredity  simply  lacked  even  moderate  capacity  and  were 
unable  to  rise  when  given  normal  opportunity.  They  be- 
longed to  the  good  solid  bourgeoisie  stock  which  fonns 
the  balance-wheel  of  modem  democracy.  But  even  grant- 
ing to  historical  rank  a  justice  which  it  does  not  have, 
admitting  no  personal  superiority  in  any  of  the  relatives 
of  these  two  men  and  others  like  them,  their  very  exist- 
ence is  a  link  in  the  proof  of  Mendelian  com.bination  in 
the  making  of  mentality.  The  great  bulk  of  the  population 
inherits  certain  factors  contributing  toward  ability.  They 
do  not  have  any  one  of  the  numerous  inherited  complexes 
which  make  a  genius  in  the  rough,  but  they  have  a  random 
sample  of  the  constituent  parts.  As  a  mere  fulfillment  of 
the  laws  of  chance,  matings  between  these  individuals 
must  occasionally  bring  together  the  happy  combination 
of  which  we  speak. 

"Whether  these  talents  lie  wrapped  in  a  napkin  or  not, 
depends,  of  course,  on  a  variety  of  circumstances.  One 
can  keep  a  good  man  down,  though  with  some  difficulty. 
It  was  probably  the  general  attitude  of  society  in  those 
particular  epochs  that  gave  a  Golden  Age  to  Greece,  a 
Eenaissance  to  Italy,  an  Elizabethan  period  to  England, 
al  Napoleonic  era  to  France,  rather  than  a  concentrated 
production  of  high  mentality.  Galton  concluded  the  ablest 
race  in  history  was  that  built  up  in  Attica  between  530  and 
430  B.C.,  when  from  45,000  free-bom  males  surviving  the 
age  of  fifty  there  came  fourteen  of  the  most  illustrious 


238         INBREEDING  AND  OUTBREEDINa 

men  of  all  time.  He  was  hardly  justified  in  this.  Men- 
tality was  then  the  order  of  the  day  in  Attica.  Accom- 
plishment was  appreciated.  Minds  of  high  order  were 
drawn  from  surrounding  countries.  The  great  poet, 
strange  as  it  may  seem,  was  valued  more  highly  than 
the  wealthy  merchant.  Such  conditions  must  account  for 
much  of  this  apparent  racial  superiority.  Further,  there 
can  be  little  question  but  that  in  this  little  settlement  there 
was  much  selective  breeding.  Had  we  the  data,  would  we 
not  find  the  Athenians  all  more  or  less  related  to  one 
another?  Had  they  not  built  up  somewhat  of  a  super- 
stock  by  inbreeding?  Endogamy  was  their  custom  (West- 
ermarck,  1901).  Marriage  with  half-sisters  was  allowable, 
and  if  an  Athenian  lived  as  husband  or  wife  with  an 
alien,  he  or  she  was  liable  to  be  sold  as  a  slave  and  have  all 
property  confiscated.  Such  inbreeding,  given  the  posses- 
sion of  desirable  characteristics  on  which  to  base  selection, 
could  hardly  fail  to  bring  results. 

In  a  sense  this  is  the  obverse  of  the  picture ;  the  reverse 
is  not  as  pleasing.  Dreary  histories  have  been  written  of 
consistently  degenerate  families,  families  with  such  a 
monotonously  infamous  record  they  are  known  through- 
out the  world.  There  are  the  Jukes,^^  an  inbred  family 
whose  record  of  pauperism,  prostitution  and  crime  has 
been  traced  for  six  generations.  There  is  the  *' Tribe  of 
Ishmael/'  a  race  of  indigent  vagrants  since  1790,  con- 
sistent in  their  wavs  of  life  no  matter  what  their  sur- 
roundings.^^2  There  is  the  Nam  family,  descendants  of  a 
racial  mixture,  Indian-white,  less  uniform  than  the  first 
two  in  their  anti-social  traits,  but  characterized  on  the 
whole  by  vagabondage,  stupidity  and  lack  of  ambition.^^ 
There  is  the  family  of  which  Poellman  recorded  709  life- 


MAN  239 

histories,  a  distressing  chronicle  of  illegitimacy  and 
pauperism.  There  is  the  Zero  clan,  a  name  fixing  well 
their  value  to  the  world,  ne'er-do-w^ells  since  the  seven- 
teenth century.^  ^^ 

We  have  no  intention  of  going  into  further  detailed 
descriptions  of  these  people.  They  are  but  examples  of  an 
heredity  which  the  world  does  not  desire.  He  who  does 
not  believe  their  characteristics  to  be  due  to  their  ger- 
minal constitution  allows  his  sympathies  to  run  riot  with 
his  reason.  Let  anyone  examine  the  published  pedigrees 
of  these  tribes  of  unregenerates,  in  the  light  of  the  facts 
discussed  in  these  pages.  He  will  find  degenerate  mating 
with  degenerate  generation  after  generation,  incest  of  the 
highest  degree,  inbreeding  of  the  most  intense  kind.  He 
will  see  segregation  in  different  strains  characterized  by 
distinct  kinds  of  degeneracy,  as  in  the  Jukes,  where  the 
descendants  of  Ada  are  prevailingly  criminal,  the  off- 
spring of  Bell  without  sexual  inhibitions,  the  progeny  of 
Effie  paupers.  He  will  find  that  outbred  lines  have  some- 
times separated  from  the  main  stock,  have  had  ambition, 
and  have  gone  out  and  made  respectable  names  for  them- 
selves. He  will  find  that  these  people  have  been  given 
chances,  have  been  removed  from  old  associations,  taken 
to  reputable  homes,  clothed,  fed,  educated  according  to 
their  capabilities,  and  have  remained — degenerate. 

Social  workers  have  been  troubled  because  more  repre- 
sentatives of  these  blood  lines  have  not  been  removed  from 
their  isolation  and  the  evil  example  it  entails,  and  been 
given  the  stimulation  of  association  mth  people  of  more 
stamina.  Let  us  be  glad  that  these  natural  experiments 
were  carried  on  as  they  were.  The  disadvantage  of 
neglect,  isolation  and  bad  example  to  the  individual  must 


240         INBREEDING  AND  OUTBREEDING 

be  admitted,  but  does  anyone  believe  that  these  families 
would  have  been  a  credit  to  the  communities  harboring 
them  if  the  environment  were  changed.  It  was  tried  many 
times  and  failed.  No !  What  happened  in  these  cases  was 
the  establishment  of  near-homozygous  races  having  a  bad 
heredity.  The  result  of  inbreeding  where  the  germ  plasm 
is  bad  stands  forth  as  a  terrible  example.  What  would 
have  happened  had  there  been  no  isolation  would  have 
been  the  contamination  of  good  blood  lines.  In  fact,  cer- 
tain illegitimate  sub-strains  in  these  clans  did  stand  out 
above  their  relatives.  Was  this  not  due  to  a  better  endow- 
ment being  brought  in  from  the  alien  male? 

The  traits  just  discussed,  at  best  mere  uselessness  or 
lack  of  capacity,  seem  to  be  somewhat  less  complex  in 
their  heredity  than  those  leading  to  marked  superiority, 
if  one  may  judge  by  the  seeming  ease  with  which  they  are 
concentrated.  Other  undesirable  mental  characteristics 
appear  to  be  still  less  complex.  These  are  f  eeble-minded- 
ness  and  the  conditions  related  to  it.  Goddard®^  has 
studied  327  family  histories  in  which  feeble-mindedness 
entered.  Somewhere  between  50  per  cent,  and  75  per  cent, 
of  these  family  trees  show  distinct  evidence  of  the  heredi- 
tary nature  of  the  defect.  There  were  144  matings  of 
feeble-minded  with  feeble-minded  producing  482  children 
of  which  all  but  6  were  feeble-minded.  These  few  excep- 
tions to  the  expectation  for  the  union  of  two  Mendelian 
recessives  may  reasonably  be  explained  by  assuming  there 
is  a  paternity  other  than  that  assigned.  Other  types  of 
mating  come  just  as  close  to  Mendelian  expectancy,  al- 
though Goddard  himself  has  failed  to  analyze  them  prop- 
erly by  not  correcting  for  the  necessary  omission  of 
heterozygous  matings  having  no  feeble-minded  children. 
We  may,  therefore,  conclude  that  feeble-mindedness  is 


MAN  241 

due  to  a  single  principal  unit  factor,  recessive  to  what  we 
may  call  normal  mentality. 

There  is  evidence,  however,  of  other  minor  factors 
which  modify  the  grade  of  feeble-mindedness,  and  con- 
siderable reason  for  feeling  that  in  some  similar  way  cer- 
tain forms  of  insanity,  epilepsy  and  other  neuroses  are  in 
some  way  related.  At  any  rate,  all  of  these  abnormalities 
are  in  many  cases  inherited  as  recessive  traits. 

Here,  then,  is  something  wholly  undesirable  which 
may  be  the  result  whenever  the  proper  unions  occur ;  and 
as  we  have  seen  how  inbreeding  tends  to  bring  out  reces- 
sive characters,  in  feeeble-mindedness  lies  a  potential 
danger.  Let  us  see  what  this  danger  is  as  regards  the 
United  States. 

It  appears  that  in  our  present  population  of  over  100,- 
000,000  there  are  something  like  300,000  persons  who  are 
feeble-minded,  epileptic  or  insane  through  an  hereditary 
defect,  a  ratio  of  3  per  1000.  How  many  of  these  detec- 
tives resulted  from  a  mating  wherein  at  least  one  of  the 
parents  was  of  the  same  type  is  a  difficult  question  and  can 
only  be  answered  with  a  rough  approximation.  The  sta- 
tistics at  present  available  are  meagre,  but  from  their 
examination  200,000  may  be  considered  to  be  above  the 
mark.  This  leaves  100,000  defectives,  then,  which  have 
been  produced  in  a  single  generation  by  the  mating  of  two 
transmitters  of  defective  mentality  who  did  not  exhibit 
such  defects  in  themselves. ^^ 

These  100,000  defectives  were  produced  during  a 
period  when  there  were  rather  less  than  20,000,000  mar- 
ried couples  of  reproductive  age,  by  parents  who  were 
both  heterozygous.  But  since  only  one-fourth  of  the 
progeny  of  such  matings  will  be  defective,  at  least  100,000 
couples  of  this  type  were  reproducing  throughout  this 

16 


242         INBREEDING  AND  OUTBREEDING 

time.  This  low  estimate  would  presuppose  the  survival 
of  four  children  per  couple  long  enough  to  have  their 
mental  status  determined,  an  assumption  which  would 
require  a  total  reproductivity  of  six  or  seven  children  per 
married  pair. 

If,  then,  out  of  20,000,000  pairs  of  married  persons, 
100,000  were  heterozygous  for  feeble-mindedness  or 
attendant  ills  on  both  sides  of  the  house,  what  would  be 
the  number  of  such  persons  in  the  general  population! 
The  problem  may  be  stated  a  little  more  clearly :  A  cer- 
tain number  of  persons  out  of  a  marriageable  population 
of  40,000,000  carry  defective  germ  cells.  If  two  of  them 
marry,  one-quarter  of  their  children  will  be  feeble-minded. 
If  100,000  such  marriages  did  occur,  what  is  the  ratio  of 
these  defect  carriers  to  normals  in  the  general  population? 

Pairing  among  defect  carriers  has  occurred  in  the 
ratio  of  1  to  200  marriages;  then  these  individuals  must 
be  present  in  the  general  population  in  the  ratio  of  1  to  14,'' 
if  no  disturbing  factors  exist. 

The  thought  that  one  person  out  of  every  fourteen 
carries  the  basis  of  serious  mental  defectiveness  in  over 
half  of  his  or  her  reproductive  cells  is  enough  to  make  the 
stoutest  heart  quake.  The  problem  of  cutting  off  defec- 
tive germ  plasm  is  not  the  theoretically  simple  one  of 
preventing  the  multiplication  of  the  afflicted;  it  is  the 
almost  hopeless  task  of  reducing  the  birth  rate  among 
the  personally  unaffected  transmitters  where  there  is 


\  200  = 


approximately  ^S.     The  probability  of  normal  mating  normal  = 

14 

(13y  =tl69,  the  probability  of  normal  mating  carrier  is  2  / 13   J_\ 
14/        196  \i4^14/ 

26    the  probability  of  two  carriers  mating  is/_l_\'==  1  . 

196,  \14/      196 


MAN  243 

little  prudential  restraint  and  consequently  a  high  repro- 
ductive rate. 

The  problem  exists  in  just  the  form  we  have  stated  it, 
but  perhaps  the  picture  has  been  overdrawn.  Although 
the  ratio  is  extremely  conservative  from  one  point  of  view 
because  of  the  low  estimate  of  defectives  and  the  tre- 
mendous birth  rate  used,  it  may  be  considerably  too  high 
by  reason  of  our  inability  to  allow  for  a  proper  selective 
marriage  rate  between  the  carriers.  Goddard,  who  has 
made  a  more  intensive  study  of  these  persons  than  any- 
one else,  is  of  the  opinion  that  the  heterozygotes  are  not 
in  the  same  class  with  pure  normals.  They  are  more  or 
less  dull,  stupid,  lacking  in  real  ability.  For  this  reason 
they  are  unquestionably  thrown  together  more  than  would 
otherwise  be  the  case.  They  tend  to  form  a  class  of  the 
population  which  weds  within  itself.  This  is  not  a  whole- 
some thing,  but  it  is  much  better  than  having  it  corrupt 
the  good  germ  plasm  of  the  country.  Although  one  can 
make  no  true  estimate,  such  a  selective  marriage  rate  may 
be  high  enough  to  revise  our  ratio  two  or  three  hundred 
per  cent.  Instead  of  1  out  of  14  being  of  this  type,  it  may 
be  only  1  out  of  28  or  42.    At  best  it  is  food  for  thought. 

Enough  has  been  said  about  the  effect  of  inbreeding  in 
man  to  show  why  the  numerous  statistical  investigations 
on  marriages  of  near  kin  have  reached  no  concordant  re- 
sults. Hundreds  of  such  investigations  have  been  made. 
The  earlier  ones  were  compiled  by  Huth,^"^  who  came  very 
near  the  truth  considering  the  state  of  knowledge  of  his 
time.  The  later  data  have  been  brought  together  by 
Westermarck,2i*  but  Westermarck  was  so  imbued  with 
preconceived  ideas  of  what  ought  to  be  true  that  he  made 
matters  more  chaotic  than  ever.    The  impossibility  of  a 


244         INBEEEBING  AND  OUTBREEDING 

correct  statistical  answer  to  tlie  problem  is  clear  if  one 
works  back  from  the  answer  given  by  the  research  on 
heredity:  '* Inbreeding  is  not  in  itself  harmful;  whatever 
effect  it  may  have  is  due  wholly  to  the  inheritance  re- 
ceived.'^ It  is  not  to  be  wondered,  therefore,  that  exami- 
nation of  the  pedigree  record  of  one  family  led  to  one 
conclusion,  and  of  another  family  to  exactly  the  opposite. 

Nothing  has  been  mentioned  about  the  effect  of  cross- 
ing in  the  human  race,  whether  such  crosses  be  narrow  or 
wide.  Such  a  discussion  belongs  more  properly  in  the 
concluding  chapter  as  it  concerns  the  race  more  than  the 
individual.  Physically  outcrossing  may  often  be  of  value 
to  the  individual,  but  there  are  reasons  to  be  discussed 
later  for  not  generalizing  hastily  in  the  matter.  What  we 
wish  to  say  in  conclusion  here  refers  still  to  inbreeding. 
It  is  this : 

Owing  to  the  existence  of  serious  recessive  traits  there 
is  objection  to  indiscriminate,  irrational,  intensive  inbreed- 
ing in  man ;  yet  inbreeding  is  the  surest  means  of  estab- 
lishing families  which  as  a  whole  are  of  high  value  to  the 
community.  On  the  other  hand,  owing  to  the  complex 
nature  of  the  mental  traits  of  the  highest  type,  the  bright- 
est examples  of  inherent  ability  have  come  and  will  come 
from  chance  mating  in  the  general  population,  the  com- 
mon people  so-called,  because  of  the  variability  there 
existent.  There  can  be  no  permanent  aristocracy  of 
brains,  because  families,  no  matter  how  inbred,  will  re- 
main variable  while  in  existence  and  will  persist  but  a 
comparatively  short  time  as  close-bred  strains.  But  he  is 
a  trifler  with  little  thought  of  his  duty  to  the  state  or  to 
himself,  who,  having  ability  as  a  personal  endowment, 
does  not  scan  with  care  the  genealogical  record  of  the 
family  into  which  he  enters. 


CHAPTER  XIII 

THE  INTEBMINGLING  OF  RACES  AND  NATIONAL 

STAMINA 

A  PROPOSAL  to  discuss  racial  mixtures  as  a  final  topic 
in  such  a  condensed  treatment  of  inbreeding  and  outbreed- 
ing as  is  presented  here,  may  be  deemed  somewhat  pre- 
sumptuous, both  because  of  the  intricacy  and  difficulty  of 
the  subject  itself  and  because  of  the  immense  amount  of 
partially  codified  knowledge  relating  to  such  matters 
which  has  been  gathered  by  anthropologists.  On  the  con- 
trary, these  very  facts  are  the  logical  reasons  for  ventur- 
ing to  indicate  how  and  where  the  conclusions  of 
experimental  genetics  can  be  applied  to  the  problems 
knoA\Ti  collectively  as  race  problems. 

The  data  of  anthropology  are  largely  those  of  the  his- 
torical type  in  which  the  control  of  variables  is  always 
uncertain  and  often  impossible.  It  is  obvious  that  the 
nature  of  the  material  to  a  certain  extent  limits  direct 
investigation  in  this  field  to  the  historical  and  the  statisti- 
cal methods  of  research.  Generally  speaking,  one  cannot 
use  man  as  the  subject  of  quantitative  laboratory  experi- 
ments. Yet  the  difficulties  involved  demand  varied 
methods  of  attack;  and  since  genetics  has  furnished  a 
satisfactory  interpretation  of  heredity  by  dealing  with  the 
lower  organisms  and  has  proved  that  the  same  mode  of 
inheritance  prevails  in  man,  it  is  inexcusable  if  the  broad 
ethnological  application  of  the  results  is  neglected.  Of 
course,  one  must  not  expect  the  impossible.  Problems  so 
complex  can  have  no  genetical  solutions  permitting 
predictions  in  individual  cases,  but  the  principles   do 

245 


246         INBREEDING  AND  OUTBREEDINO 

give  information  based  upon  the  law  of  averages  which 
has  some  importance. 

Man  is  a  single  species  if  one  may  judge  from  the 
interf  ertility  and  the  blood  chemistry  of  existing  peoples, 
but  mankind  are  not  all  brothers  in  spite  of  this  oft- 
repeated  laoonicism  of  idealists  and  radicals;  through 
some  300,000  years  of  evolution  the  relationship  between 
the  extremes  is  rather  vague.  During  this  period  the 
black  race,  the  yellow  race,  the  white  race — three  well- 
marked  varieties  of  the  species — have  come  into  existence ; 
and  the  total  number  of  heritable  variations  differentiat- 
ing sub-races  and  individuals  is  almost  incalculable. 
Naturally,  the  selective  agents  concerned  in  this  process 
of  segregation  were  numerous ;  but  isolation  in  a  broad 
sense,  necessitating  as  it  did  a  variety  of  group  struggles 
for  existence  amid  different  enviroimients,  probably  may 
be  regarded  as  the  factor  chiefly  responsible.  In  the  im- 
mediate past,  however,  a  short  period  as  evolutionary 
time  is  marked,  there  has  been  an  increasingly  swift 
reversal  of  this  process  of  racial  separation.  History,  in 
fact,  has  been  hardly  more  than  a  record  of  successive 
race  migrations  with  the  inevitable  mingling  of  the  con- 
queror vnth  the  conquered. 

With  the  twentieth  century  the  world  enters  a  new 
phase  of  development.  Witliin  a  single  generation  man 
has  reached  out  and  grasped  the  mastery  of  his  environ- 
ment. Space  has  been  annihilated  with  the  telegraph  and 
telephone,  the  railway,  the  steamboat,  the  submarine,  the 
aeroplane.  As  a  result  of  this  freedom  of  communication 
there  will  be  even  more  colonization  until  the  limit  of  the 
food  supply  is  reached ;  and  then,  a  stationary  population, 
through  an  increased  death  rate  or  a  decreased  birth  rate. 


INTERMINGLING  OF  RACES  247 

It  is  unfortunate,  in  view  of  the  facts  in  the  case,  that 
many  should  still  scoff  at  the  conclusions  of  Malthus  on 
the  subject  of  population,  reached  a  century  ago.  The 
impossibility  of  the  food  supply  keeping  pace  with  an  un- 
checked natural  increase  of  population  is  a  truism  which 
cannot  be  glossed  over  by  pointing  to  the  ingenuity  of 
man  in  applying  mechanics  to  agriculture.  The  truth  is 
that  the  world  is  approaching  a  population  limit  faster 
even  than  Malthus  supposed,  and  the  result  of  applying 
new  methods  to  field  culture  is  merely  to  exploit  the 
natural  fertility  of  the  soil  at  a  higher  rate.  The  sup- 
posed increase  in  the  amount  of  food  is  illusorj^  In  the 
United  States,  naturally  the  richest  country  on  the  globe, 
the  per  capita  production  of  all  the  important  meat 
animals  and  some  of  the  great  agricultural  crops 
is  decreasing. 

At  present  the  situation  is  this :  China,  having  reached 
the  limit  of  her  food  supply,  and  having  little  or  no 
foreign  trade,  has  become  stationary  in  population.  Large 
portions  of  Europe  and  the  country  of  Japan  have  reached 
the  limit  of  sustenance  within  themselves,  but  are  increas- 
ing at  a  rate  of  from  ten  to  fifteen  per  thousand  annually 
because  their  commerce  is  such  as  to  permit  importation 
to  supply  the  deficit.  Australia  and  New  Zealand  and 
other  parts  of  Asia  and  Europe  are  increasing  at  a  rate 
which  neither  their  agriculture  nor  their  commerce  can 
long  sustain.  The  Americas  and  Africa  are  left  as  the 
great  centres  of  colonization.  Each  will  support  a  large 
additional  number  of  people ;  but  when  they  have  reached 
their  limit,  and  that  limit  will  come  within  a  very  few 
centuries — three  at  most — each  country,  or  at  least  each 
continent,  must  support  its  own  population. 


248         INBREEDING  AND  OUTBEEEDING 

As  an  outcome  of  these  conditions,  the  world  faces  in- 
creasing amounts  of  race  amalgamation,  and  there  is 
naturally  an  acute  interest  in  race  problems.  The  greater 
part  of  this  interest  is  due  to  prejudice  arising  from 
racial  and  national  arrogance.  Normally  each  sub-race 
believes  implicitly  in  its  own  superiority  and  hopes  for 
continued  increase  and  ultimate  survival.  Perhaps  such 
prejudice  prevents  any  wholly  objective  discussion  of  the 
matter.  But  apart  from  desires  and  hopes  concerning 
racial  domination,  it  ought  to  be  possible  to  set  forth  the 
facts  as  they  are  and  to  determine  roughly  what  ought  to 
occur  under  given  conditions. 

In  order  that  there  shall  be  no  misunderstanding  in 
regard  to  the  premises  taken,  let  us  first  consider  the 
classification  of  man  from  the  anthropological  and  from 
the  genetic  viewpoints. 

Anthropologists  have  been  confronted  with  the  very 
difficult  task  of  classifying  existent  peoples  both  with  the 
view  of  furnishing  a  useful  nomenclature  and  for  the  pur- 
pose of  solving  problems  of  descent.  They  have  recog- 
nized the  insubstantial  character  of  a  language  or  a 
nationality  basis  and  have  founded  their  systems  on 
physical  traits.  Even  these,  head  form,  hair  shape,  skin 
color,  stature,  and  so  on,  have  been  freely  acknowledged 
to  be  less  satisfactory  than  might  be  desired.  Neverthe- 
less the  systems  in  vogue  have  been  serviceable  in  many 
ways,  and  it  is  only  when  they  are  used  as  quantitative 
measures  of  ancestry  that  the  geneticist  is  inclined  to  raise 
certain  objections. 

The  difficulties  of  the  anthropologist  are  relatively 
much  greater  than  those  of  other  systematists.  Inter- 
group  sterility  is  a  great  aid  to  botanists  and  zoologists. 


INTERMINGLING  OF  RACES  249 

In  general  their  taxonomists  have  had  only  to  differen- 
tiate strains  which  do  not  interbreed.  The  mission  of  the 
ethnologist  may  be  compared  rather  to  that  of  the  agri- 
culturist who  is  called  upon  to  produce  a  usable  classifica- 
tion of  the  numerous  strains  of  a  variable  domesticated 
species,  such  as  cattle  and  swine,  or  even  wheat  and 
maize.  He  must  for  the  sake  of  convenience  make  a  mor- 
phological grouping  that  is  non-existent  in  physical  fact. 
He  does  this  by  taking  advantage  of  isolation;  without 
isolation  it  is  impossible. 

An,  appreciation  of  Mendelian  inheritance  shows  the 
fallacy  involved  in  making  such  a  system  a  basis  for  trac- 
ing ethnic  relationships.  An  immense  number  of  heredi- 
tary variations  have  occurred  in  man.  Some  can  be 
described  by  the  main  structural  changes  they  effect,  some 
cannot.  Some  distinct  changes,  such  as  eye  color,  have  a 
very  simple  method  of  inheritance.  They  are  the  mark 
of  single-factor  differences  in  the  germ  plasm.  Other 
ichanges,  such  as  those  expressed  in  stature  and  skull 
form,  appear  to  be  controlled  by  numerous  factors.  There 
are  even  numerous  factor  changes  which  seem  to  produce 
no  visible  effect  on  the  individual  and  whose  existence  can 
be  shown  only  by  crossing.  For  example,  it  may  be 
assumed  with  considerable  confidence  that  individuals  can 
have  the  same  cephalic  index  and  yet  differ  by  several 
hereditary  factors  whose  chief  functions  are  the  control 
of  this  character.  At  least  such  cases  have  been  found  in 
other  species  and  there  is  no  reason  for  supposing  they  do 
not  occur  in  man. 

Now  these  various  physical,  physiological  and  psychic 
characters  are  controlled  by  factors  transmitted  alter- 
natively.   They  may  be  linked  in  various  manners,  it  is 


250         INBEEEDING  AND  OUTBEEEDING 

true,  but  they  are  presumably  Mendelian.  Consequently 
one  must  be  very  cautious  about  drawing  genetic  conclu- 
sions in  the  human  race  based  upon  the  possession  of 
particular  traits,  in  the  absence  of  proof  of  a  long-con- 
tinued isolation.  Long  isolation,  it  must  be  assumed, 
aided  in  segregating  some  well-marked  human  sub- 
species. It  may  serv^e  a  purpose  to  continue  to  accept  cer- 
tain of  these  types  as  implied  in  the  terms,  white,  yellow 
and  black  races.  Yet  one  must  not  forget  that  real  isola- 
tion belongs  to  past  epochs.  There  has  been  no  small 
amount  of  interbreeding  between  even  these  main  types, 
and  the  magnitude  of  the  interbreeding  between  sub-races 
is  largely  a  matter  of  historical  record.  Traits  originally 
characteristic  of  certain  peoples  because  of  isolation  and 
the  consequent  inbreeding  have  been  shifted  back  and 
forth,  combined  and  recombined.  It  is  positively  mis- 
leading, therefore,  to  classify  Englishmen  as  resembliiig 
Danish,  Norman^  Pictic,  Celtic  or  Bronze  Age  types,  as  is 
done  in  more  than  one  work  of  authority.  Even  if  it  were 
known  what  the  average  values  of  the  various  characters 
of  these  early  strains  were,  there  is  little  reason  for  be^ 
lieving  that  a  present-day  individual  bearing  one  or  two 
particularly  striking  traits  should  be  felt  to  hold  any 
closer  relationship  to  the  strain  in  which  these  traits  are 
supposed  to  have  arisen  than  his  neighbors  who  are  with- 
out them.  He  may  have  outstanding  characters  which 
were  once  peculiar  to  a  comparatively  pure  race ;  but  he 
probably  carries  these  characters  as  a  mere  matter  of 
Mendelian  recombination.  It  is  wholly  possible,  for  ex- 
ample, that  a  tall,  blue-eyed,  dolichocephalic  Frenchman 
really  possesses  less  of  the  so-called  Nordic  factors  than  a 
short,  dark-eyed  round-head. 


INTERMINGLING  OF  RACES  251 

One  other  matter  to  be  kept  in  mind  in  this  connection 
is  the  impossibility  of  knowing  what  factors  have  survived 
and  what  have  perished.  The  great  differences  between 
individuals  in  inherent  traits  both  physical  and  mental, 
make  it  probable  that  even  within  a  race  the  average 
capacity  of  some  strains  is  greater  than  others.  This 
seems  a  fair  deduction  after  making  all  due  allowances 
for  changes  in  the  spirit  of  the  times  which  accelerate  or 
retard  the  development  of  natural  ability.  Now  we  do  not 
know  and  cannot  know  how  the  hereditary  factors  existent 
to-day  compare  with  those  existent  2000  years  ago.  Selec- 
tion within  the  population  is  an  invariable  concomitant  of 
human  existence.  There  is  a  selective  death  rate,  selec- 
tive mating,  selective  fertility,  each  influenced  by  many 
conditions.  These  selective  agencies  do  not  remain  the 
same,  nor  does  the  material  upon  which  they  work.  We 
do  not  know,  for  example,  whether  the  most  desirable 
germ  plasm  of  Greece,  of  Rome,  of  Mediasval  Europe,  has 
been  passed  on  or  has  ceased  to  exist.  The  world  has 
received  a  great  legacy  in  the  creative  production  of  the 
master  men  of  the  past ;  that  it  is  their  heir  in  physical 
fact  is  not  so  certain.  In  the  strictly  biological  sense,  i.e., 
in  the  material  basis  of  heredity,  the  world  may  be  better 
or  it  may  be  worse  than  it  was  in  the  time  of  Pericles. 
This  phase  of  the  subject  is  mentioned  because  one  often 
hears  comments  on  the  degeneracy  of  certain  nations  who 
have  had  periods  of  enviable  greatness.  Social  condi- 
tions may  be  the  cause,  but  it  should  not  be  forgotten 
that  they  are  not  the  sole  possible  cause.  Essential 
hereditary  factors  may  have  been  cut  off,  may  have  been 
wholly  eliminated. 

These  genetic  ideas  of  race  heredity  among  mankind 


252         INBREEDING  AND  OUTBREEDING 

are,  we  believe,  fundamental.  They  give  a  clue  as  to  what 
has  happened  in  the  racial  mixtures  of  the  past,  and 
enable  one  to  visualize  more  clearly  the  probable  result  of 
the  intense  race  amalgamation  to  be  expected  in  the 
twentieth  century. 

The  world  faces  two  types  of  racial  combination :  one 
in  which  the  races  are  so  far  apart  as  to  make  hybridiza- 
tion a  real  breaking-dowTi  of  the  inherent  characteristics 
of  each ;  the  other,  where  fewer  differences  present  only 
the  possibility  of  a  somewhat  greater  variability  as  a 
desirable  basis  for  selection.  Roughly,  the  former  is  the 
color-line  problem;  the  latter  is  that  of  the  White  Melt- 
ing Pot,  faced  particularly  by  Europe,  North  America 
and  Australia. 

The  genetics  of  these  two  kinds  of  racial  intermixture 
is  as  follows:  Consider  first  a  cross  between  two  ex- 
tremes, typical  members  of  the  white  and  of  the  black  race. 
In  the  first  generation  the  individuals  show  a  notable 
amount  of  heterosis,  indicating  differences  in  a  large 
number  of  hereditary  factors.  They  are  intermediate  in 
hair  form,  skin  color,  head  shape,  and  various  other 
physical  attributes,  in  mental  capacity,  and  in  psychical 
characters  in  general ;  although  they  show  extraordinary 
physical  vigor.  In  later  generations  segregation  and  re- 
combination in  many  of  these  characters  can  be  traced 
with  little  difficulty ;  but  if  one  describes  the  descendants 
of  the  cross  as  a  population,  or  even  the  total  character- 
istics of  a  single  individual,  fluctuation  around  the  aver- 
age of  the  two  original  races  is  still  the  rule.  There  may 
be  an  approach  to  the  head  form  of  one  race  combined 
with  the  skin  color  of  the  other,  an  approximation  of  the 
hair  of  the  one  coupled  with  the  other's  stature;  never- 


INTERMINGLING  OF  RACES  253 

theless,  there  is  little  likelihood  of  an  individual  return  to 
the  pure  type  of  either  race.  The  difficulties  involved  are 
those  described  in  Chapter  VII.  The  races  differ  by  so 
many  transmissible  factors,  factors  which  are  probably 
linked  in  varied  ways,  that  there  is,  practically  speaking, 
no  reasonable  chance  of  such  breaks  in  linkage  occurring 
as  would  bring  together  only  the  most  desirable  features, 
even  supposing  conscious  selection  could  be  made.  And 
selection  is  not  conscious.  Breeding  for  the  most  part  is 
at  random.  The  real  result  of  such  a  wide  racial  cross, 
therefore,  is  to  break  apart  those  compatible  physical  and 
mental  qualities  which  have  established  a  smoothly  oper- 
ating whole  in  each  race  by  hundreds  of  generations  of 
natural  selection. 

If  the  two  races  possessed  equivalent  physical  char- 
acteristics and  mental  capacities,  there  would  still  be 
this  valid  genetical  objection  to  crossing,  as  one  may 
readily  see.  Butin  reality  the_negro  is  inferior  toJ;he 
white.^®^  This  is  not  hypothesis  or  supposition;  it  is  a 
crude  statement  of  actual  fact.  The  negro  has  given  the 
world  no  original  contribution  of  high  merit.  By  his  own 
initiative  in  his  original  habitat,  he  has  never  risen. 
Transplanted  to  a  new  environment,  as  in  the  case  of 
Haiti,  he  has  done  no  better.  In  competition  with  the 
white  race,  he  has  failed  to  approach  its  standard.  But 
because  he  has  failed  to  equal  the  white  man's  ability,  his 
natural  increase  is  low  in  comparison.  The  native  popu- 
lation of  Africa  is  increasing  very  slowly,  if  at  all.  In 
the  best  environment  to  which  he  has  been  subjected,  the 
United  States,  his  ratio  in  the  general  population  is 
decreasing.  His  only  chance  for  an  extended  survival 
is  amalgamation. 


254         INBREEDING  AND  OUTBREEDING 

The  United  States  has  been  confronted  by  this  grave 
question  for  some  time.  In  Africa  it  has  hardly  yet  come 
to  the  fore,  but  within  three  generations  it  will  be  recog- 
nized as  the  political  and  economic  problem.  What  the 
solution  will  be,  no  one  knows.  It  seems  an  unnecessary 
accompaniment  to  humane  treatment,  an  illogical  exten- 
sion of  altruism,  however,  to  seek  to  elevate  the  black 
race  at  the  cost  of  lowering  the  white.  And  the  state- 
ment is  made  with  all  due  regard  to  the  fact  that  there 
are  certain  desirable  characteristics  existent  in  the  black 
race,  and  that  unquestionably  the  two  races  overlap  in 
general  inlierent  capacity.  The  white  race  as  a  whole  is 
not  equal  to  the  black  race  in  resistance  to  several  serious 
diseases,  as  the  medical  records  of  the  United  States 
army  show.  The  two  strains  have  built  up  disease  resist- 
ance along  different  lines,  and  the  addition  of  both 
sets  of  immunity  factors  would  be  desirable.  But  the 
practical  attainment  of  such  a  benefit,  given  the  genetic 
premises,  is  so  improbable  as  to  be  negligible,  apart  from 
other  considerations. 

What  would  be  the  result  of  racial  intermixture  be- 
tween the  yellow  and  the  white  is  not  so  certain.  Both 
races  have  produced  high  types.  Can  one  say  that  either 
is  on  the  whole  the  better  ?  The  Chinese,  the  development 
of  very  early  tribal  mixtures,  have  had  a  great  productive 
period.  In  a  sense  their  productivity  has  decreased,  yet 
their  germ  plasm  is  unquestionably  good.  The  Japanese, 
the  result  of  a  much  later  racial  amalgamation,  have  de- 
veloped into  a  wonderful  people.  Whether  it  is  fair  to 
say  the  white  race  is  the  greater  because  in  the  past  two 
centuries  they  have  made  such  wonderful  contributions 
to  civilization  is  a  question.     The  contributions  of  the 


INTERMINGLING  OF  RACES  255 

yellow  race  4000  years  ago  were  as  marvellous  in  their 
time.  Yellow-white  amalgamation  may  not  be  fraught 
with  the  evil  consequences  in  the  wake  of  the  yeUow- 
blaek  and  the  white-black  crosses.  At  the  same  time  it 
should  be  pointed  out  that  the  Caucasian  and  the  Mon- 
golian are  far  apart  in  descent,  and  the  advantages  to  be 
gained  by  either  in  thus  breaking  up  superior  hereditary 
complexes  developed  during  an  extended  past  are  not 
clear.  At  any  rate,  there  seems  to  be  no  prospective  bene- 
fit to  the  superior  yellow  peoples  in  mating  with  some 
of  the  inferior  existent  whites,  and  no  presumable  good 
to  the  superior  white  in  intermingling  with  the  poorer 
yellow  offshoots,  as  has  been  done  to  the  south  of  the 
United  States. 

Our  first  conclusion  may  be  said  to  be  a  decision 
against  the  union  of  races  having  markedly  different 
characteristics — particularly  when  one  is  decidedly  the 
inferior.  Through  the  operation  of  the  laws  of  heredity 
such  unions  tend  to  break  apart  series  of  character  com- 
plexes which  through  years  of  selection  have  proved  to 
be  compatible  with  each  other  and  with  the  persistence 
of  the  race  under  the  environment  to  which  it  has  been 
subjected.  Because  of  the  transmission  of  factors  in 
linked  groups,  the  low  probability  of  obtaining  a  single 
recombination  equal  or  superior  to  the  average  of  the 
better  race  does  not  warrant  the  production  of  multitudes 
of  racial  mediocrities  which  such  a  mixture  entails. 

Our  second  thesis  is  seemingly  paradoxical.  It  asserts 
that  the  foundation  stocks  of  races  which  have  impressed 
civilization  most  deeply  have  been  produced  by  inter- 
mingling peoples  who  through  one  cause  or  other  became 
genetically  somewhat  unlike.    Theoretically,  this  theorem 


256         INBKEEDING  AND  OUTBREEDING 

is  not  difficult  to  develop.  Whatever  the  causes  of  racial 
separation  under  the  isolation  characteristic  of  former 
times,  peoples  did  come  to  have  a  rather  narrow  vari- 
ability. They  were,  one  might  say,  homozygous  for  cer- 
tain traits.  These  traits  naturally  differed  in  their  value. 
There  were  great  peoples,  mediocre  peoples,  and  wretched 
peoples.  But  each  was  more  or  less  standardized.  "When 
there  came  occasion  for  these  standardized  peoples  dii- 
fering  in  their  transmissible  characters  to  intermingle, 
great  variability  was  produced;  and  if  the  differences 
were  not  too  great,  the  chances  were  high  that  valuable 
character  combinations  would  come  to  light.  Later 
through  the  close  breeding  due  to  the  marriage  selection 
which  always  develops  within  human  society,  the  ten- 
dency was  again  to  produce  purer  strains  characterized 
differently ;  but  without  the  chance  of  repeated  Mendelian 
recombinations  the  probability  of  establishing  superior 
strains  was  small. 

*•  This  hypothesis,  developed  wholly  from  a  considera- 
tion of  the  genetic  facts,  is  not  refuted  by  ethnological 
data.  Thus,  if  one  considers  the  peoples  of  Europe,  he 
finds  high  civilizations,  invariably  following  the  migra- 
tion of  that  ancient  race  or  mixture  of  races  termed 
Aryan,  a  people  of  whom  there  is  now  only  circumstantial 
evidence.  It  was  manifestly  not  mere  hybridization 
which  brought  results  of  outstanding  value,  however,  but 
hybridization  of  good  strains  not  too  widely  differen- 
tiated, followed  by  periods  of  more  or  less  intensive  in- 
breeding. This  is  a  reasonable  deduction  from  the 
rapidity  with  which  European  and  particularly  North 
European  culture  has  outstripped  that  of  Central  and 
Southern  Asia. 


INTERMINGLING  OF  RACES  257 

The  difficulty  with  using  these  data  as  actual  support 
of  the  hypothesis  under  consideration  comes  from  the 
fact  that  the  amount  of  hybridization  appears  to  be  about 
the  same  in  various  peoples  who  differ  greatly  in  their 
contributions  to  civilization.  This  may  mean  that  the 
strains  supposedly  lower  in  ability  have  potentialities  not 
yet  realized,  or  it  may  mean  inherent  differences  in  the 
original  constituent  parts.  This  much  seems  to  be  true, 
however.  The  great  individuals  of  Europe,  the  leaders 
in  thought,  have  come  in  greater  numbers  from  peoples 
having  very  large  amounts  of  ethnic  mixture.  Even  the 
Scandinavians,  a  relatively  pure  strain  of  the  stock  to 
which  much  of  the  greatness  of  Germany,  England  and 
even  France  is  supposed  to  be  due,  have  been  somewhat 
behind  these  peoples  in  the  production  of  constructive 
leaders.  Is  it  not  a  fair  assumption  that  the  backward- 
ness of  Spain  and  Ireland  is  due  to  their  relative  isola- 
tion! Is  it  not  because  the  waves  of  migration  were 
nearly  spent  before  they  reached  these  lands'  ends? 

Contrast  the  people  in  the  United  Kingdom,  more 
particularly  the  natives  of  the  south  and  west  of  Ireland, 
with  those  of  Scotland  and  England.  In  proportion  to 
their  numbers  no  modern  people  has  approached  the  Eng- 
lish and  Scotch  in  number  of  illustrious  men  or  in  height 
of  creative  ability  except  the  French ;  the  true  Irish  have 
hardly  a  single  individual  meriting  a  rank  among  the 
great  names  of  history,  or  a  contribution  to  literature, 
art,  or  science  of  first  magnitude. 

The  Irish  are  supposed  to  have  arisen  somewhat  as 
follows  (Ripley,^^^  MacNamara  ^^^).    In  the  early  quater- 
nary period,  western  Europe  and  northern  Africa  were 
occupied  by  an  extremely  low  type  of  being  of  Mongolian 
17 


258         INBREEDING  AND  OUTBREEDING 

antecedents,  the  Iberian  race.  Until  the  neolithic  period 
these  tribes  were  the  only  inhabitants  of  the  British  Isles. 
During  the  Stone  Age,  however,  there  is  evidence  of  the 
presence  of  a  second  Mongoloid  race,  the  Turanian.  Just 
before  the  Bronze  Age  an  Aryan  stock,  the  Celts,  invaded 
Britain  and  Ireland.  These  people  came  from  the  south 
— France  or  Spain.  Probably  they  were  originally  close 
relatives  of  the  Aryans  who  migrated  from,  Asia  to  the 
northwest  and  by  intermingling  with  the  natives  and  de- 
veloping as  they  went,  formed  the  vigorous  Teutonic 
Aryan  or  Nordic.  But  the  southern  migration  of  Aryans 
met  very  different  tribes  on  their  journey,  producing  in 
the  Celts  a  somewhat  inferior  stock.  However  this  may 
be,  the  original  Celtic  horde  probably  did  not  make  a  great 
impression  on  the  racial  character  of  the  Irish ;  something 
which  also  may  be  said  of  the  second  Celtic  strain,  more 
highly  civUized  and  warlike  than  the  original  visitors, 
which  entered  Ireland  during  the  Bronze  Age.  This  later 
stream  of  invasion  continued  over  a  long  period  for  the 
island  was  not  completely  subjugated  until  well  into  the 
fifth  century;  but  the  intruders  came  as  conquerors  of  a 
higher  social  order  whose  social  ideal  was  to  keep  their 
stock  uncontaminated  with  the  blood  of  the  native  race. 

The  Norsemen,  Nordic  Aryans,  attempted  many  times 
to  gain  possession  of  Ireland  between  the  ninth  and  the 
fourteenth  centuries,  but  were  unsuccessful,  and  as  the 
Romans  and  the  Saxons  never  attempted  to  invade  Ire- 
land, the  land  won  by  the  Celtic  chiefs  remained  in  the 
hands  of  their  direct  descendants  until  1654,  when  Crom- 
well confiscated  it,  and  either  killed  or  reduced  them  to 
the  condition  of  laborers. 

The  present  inhabitants  of  Ireland,  then,  with  the  ex- 


INTERMmGLINa  OF  RACES  259 

ception  of  the  northern  counties,  where  there  is  a  consid- 
erable proportion  of  English  and  Scotch,  are  in  the  main 
descended  from  two  savage  tribes,  the  Iberian  and  Turan- 
ian, both  probably  Mongolian  admixtures,  with  the  addi- 
tion of  some  blood  of  the  conquering  and  ruling  Celtic 
Aryans,  who  genetically  must  have  been  more  or  less  in- 
tercrossed with  Iberian  and  Turanian  tribes  by  the  time 
they  reached  the  island.  Comparatively  close  inter- 
breeding for  at  least  ten  centuries  has  produced  the 
Irish  of  to-day. 

The  original  population  of  Britain,  as  of  Ireland,  was 
Iberian  overlaid  with  Turanian  in  the  north  and  some 
other  Mongoloid  tribes  in  the  south.  These  races  and  the 
types  they  produced  by  intermarriage  formed  the  bulk  of 
the  population  even  up  to  the  time  of  Caesar  *s  invasion, 
though  the  ruling  classes  were  probably  Celtic  Aryans. 
The  Romans,  anthropologists  tell  us,  made  little  change 
in  the  racial  character  of  the  inhabitants ;  but  this  state- 
ment must  be  taken  with  some  reservation.  It  is  hardly 
likely  that  the  large  garrisons  kept  by  the  Roman  Empire 
for  500  years  left  no  descendants.  As  a  population,  per- 
haps, the  racial  characteristics  were  not  changed  to  a 
noticeable  degree;  nevertheless,  a  comparatively  few 
thousand  persons  with  Roman  blood  may  have  had  some 
considerable  effect  on  the  nation  as  individuals,  and  the 
probable  presence  of  this  germ  plasm  must  not  be  counted 
as  negligible.  However  this  may  be,  the  matter  is  per- 
haps of  little  importance  as  far  as  the  Scotch  and  Eng- 
lish of  to-day  are  concerned,  for  the  greater  part  of  these 
early  peoples,  as  well  as  the  descendants  of  the  Jutes  who 
entered  the  country  in  the  fifth  century,  were  extermi- 
nated by  the  Nordic  Aryans  that  invaded  the  country 


260         INBREEDING  AND  OUTBREEDING 

under  the  various  names  of  Saxons,  Angles  and  Franks 
between  the  sixth  and  tenth  centuries.  There  was  no 
great  racial  change  made,  then,  when  the  Danes  con- 
quered the  country  in  the  early  part  of  the  eleventh 
century,  or  when  William  the  Conqueror  brought  over 
his  Normans  of  the  same  stock  in  the  latter  part. 

The  main  point  we  wish  to  bring  out  is  that  England 
and  Scotland  are  to-day  inhabited  by  an  extremely  vari- 
able people,  made  so  by  innumerable  crosses  into  which 
entered  the  blood  of  many  Nordic  Aryans  who  differed 
from  each  other  in  some  degree.  It  makes  no  difference 
whether  there  is  some  variance  among  ethnologists  as  to 
the  exactitude  of  the  racial  history.  That  is  not  essential 
and  one  need  not  quibble  about  it.  The  fact  remains  that 
the  English  and  Scotch  have  a  generally  high  civic  value 
and  are  extremely  variable.  They  produce  genius  and 
they  produce  wretchedness  as  the  natural  result  of  the 
recombination  of  these  variations.  Selections  made  from 
the  best  of  these  segregates  have  given  the  United  States 
names  of  which  one  may  well  be  proud ;  selections  made 
from  the  other  extreme  have  furnished  several  of  the 
undesirable  strains  described  previously  under  the 
pseudonyms  Nam,  Juke,  etc. 

The  Irish,  on  the  other  hand,  and  the  same  might  be 
said  of  some  other  isolated  types,  are,  much  purer 
from  the  genetic  standpoint.  Is  there  not  some  reason 
for  attributing  to  this  comparative  purity,  to  this  lack 
of  flexibility,  their  present  position  as  a  race  and 
as  individuals? 

A  case  similar  to  that  of  England  and  Scotland  might 
be  made  out  of  France  and  for  Germany,  though  France 
has  perhaps  a  greater  proportion  of  the  blood  of  the 


INTERMINGLING  OF  RACES  261 

Alpine  and  Mediterranean  peoples  than  even  the  south  of 
England  and  Wales.  But  even  so,  the  racial  differences 
have  not  been  so  great  but  that  France  has  become  one 
people,  with  all  the  chances  for  good  held  by  a  compara- 
tively small  united  nation,  when  an  amount  of  close 
breeding  has  taken  place  sufficient  to  bring  out  the  in- 
herent possibilities. 

Another  people,  great  in  their  influence  on  the  civil- 
ization of  western  Europe,  are  the  Jews.  They  should 
not  be  overlooked  in  this  connection,  because  of  the  mis- 
taken idea  that  they  form  a  pure  race  of  narrow  vari- 
ability characterized  by  fixed  traits.  Nothing  is  further 
from  the  truth.  If  it  were  the  truth  it  might  be  questioned 
whether  the  Jew  would  have  produced  the  great  number 
of  illustrious  men  who  must  in  all  fairness  be  credited 
to  them. 

The  very  term  race  applied  to  the  Jew  is  a  misnomer. 
There  is  no  more  a  Jewish  race  than  there  is  an  English 
race.  The  fiction  has  been  kept  up  because  of  a  cult  of 
racial  purity  in  their  religion.  As  a  matter  of  fact,  the 
early  Jewish  people  arose  from  complex  crosses  in  whicli 
at  least  three  different  stocks  entered:  the  Arabs,  the 
Assyrioides  or  Hittites  and  the  Aryan  Amorites.  More 
or  less  inbreeding  did  follow  before  their  dispersal,  but 
that  great  racial  variability  must  have  remained  at  the 
most  nationalistic  period  of  their  history  any  student  of 
history  knows.  After  their  dispersal  there  was  a  period 
of  proselyting  which  broadened  their  possibilities.  Later, 
moving  into  every  part  of  Europe,  they  mixed  with  tlie 
people  with  whom  they  sojourned  to  a  very  considerable 
extent,  though  keeping  up  the  while  the  religious  ideal  of 
racial  purity.   In  actual  fact  the  Spanish  Jew,  the  German 


262         INBEEEDING  AND  OUTBEEEDING 

Jew  and  the  English  Jew  are  to-day  scarcely  more  alike 
than  the  Spaniard,  the  German  or  the  Englishman ;  but  the 
practical  results  of  their  religious  beliefs,  since  they  were 
attained  but  partially,  have  been  good  in  the  genetic  sense, 
for  a  sufficient  amount  of  inbreeding  has  prevailed  to 
bring  out  the  possibilities  inherent  in  the  combinations 
made.  Civically  this  conventional  isolation  has  worked  to 
the  disadvantage  both  of  the  people  themselves  and  of  the 
State  in  which  they  held  citizenship,  and  at  present  it 
would  unquestionably  be  better  from  all  points  of  view  for 
them  to  follow  the  advice  of  some  of  their  broader  minded 
leaders  in  the  United  States  and  abandon  it,  since  their 
own  variability  has  become  so  great  as  to  make  even  a 
theoretically  fixed  policy  of  intraracial  marriage  undesir- 
able. An  alliance  of  a  Jew  of  high  capacity  and  proved 
worth  with  a  ^^Nam*^  or  a  ''Juke''  of  his  own  religion  is 
no  more  to  be  commended  than  a  similar  alliance  among 
those  of  other  faiths. 

These  three  illustrations  must  suffice  as  anthropologi- 
cal support  of  the  point  we  have  endeavored  to  emphasize. 
In  themselves  they  are  not  particularly  convincing,  it 
must  be  admitted.  Such  data  can  never  be  used  as  critical 
tests  of  biological  theory.  At  the  same  time,  when  con- 
sidered carefully  in  the  light  of  the  purely  genetic  facts 
presented,  it  seems  to  us  one  must  assent  to  the  general 
truth  of  the  theses  laid  down.  Man,  like  other  organic 
species,  has  varied  markedly  in  hereditary  characters. 
Eaces  have  arisen  which  are  as  distinct  in  mental  capacity 
as  in  physical  traits.  These  transmissible  qualities  are 
governed  by  germinal  factors  and  these  factors  are 
passed  on  to  succeeding  generations  by  the  same  precise 
laws  that  have  been  discussed  in  the  preceeding  pages. 


INTERMINGLING  OF  RACES  263 

This  being  true,  racial  crossing  may  be  desirable  or  un- 
desirable, depending  first  on  whether  the  stocks  concerned 
possess  a  preponderance  of  desirable  characteristics,  and 
second,  on  whether  they  are  extremely  differentiated  or 
not.  It  may  be  questioned  whether  all  existing  peoples 
do  not  possess  some  desirable  traits  and  hence  hold  out 
the  possibility  of  the  production  of  some  superior  indi- 
viduals when  crossed  with  presumably  superior  stock. 
Nevertheless,  even  as  in  breeding  for  quality  in  domestic 
animals,  the  frequency  with  which  the  superior  individual 
is  obtained  by  such  a  procedure  is  so  low  that  economi- 
cally radical  experiments  are  unwise.  Given  some  pre- 
sumption of  equally  desirable  contribution  in  the  union, 
the  wisdom  of  a  particular  racial  cross  is  governed  by  the 
number  of  hereditary  differences  brought  together.  The 
hybridization  of  extremes  is  undesirable  because  of  the 
improbability  of  regaining  the  merits  of  the  originals,  yet 
hybridization  of  somewhat  nearly  related  races  is  almost  a 
prerequisite  to  rapid  progress,  for  from  such  hybridiza- 
tion comes  that  moderate  amount  of  variability  which  pre- 
sents the  possibility  of  the  super-individual,  the  genius. 
To  produce  greatness  a  nation  must  have  some 
wretchedness,  for  such  is  the  law  of  Mcndelian  recom- 
bination; but  the  nation  that  produces  wretchedness 
is  not  necessarily  in  the  way  of  producing  greatness. 
There  must  be  racial  mixture  to  induce  variability,  but 
these  racial  crosses  must  not  be  too  wide  else  the  chances 
are  too  few  and  the  time  required  is  too  great  for  the 
proper  recombinations  making  for  inherent  capacity  to 
occur.  Further,  there  must  be  periods  of  more  or  less 
inbreeding  following  racial  mixtures,  if  there  is  to  be  any 
high  probability  of  isolating  desirable  extremes.    A  third 


264         INBEEEDING  AND  OUTBREEDING 

essential  in  the  production  of  racial  stamina  is  that  the 
ingredients  in  the  Melting  Pot  be  sound  at  the  beginning, 
for  one  does  not  improve  the  amalgam  by  putting  in  dross. 

May  we  consider,  in  conclusion,  the  bearing  of  these 
facts  upon  the  problem  of  this  particular  country,  the 
United  States  of  America?  The  United  States  at  one 
time  was  the  Mecca  of  the  politically  oppressed.  Free- 
dom-loving people  of  good  lineage  and  worthy  attain- 
ments came  to  its  shores.  Now,  except  for  temporary 
abatement  of  immigration  due  to  the  world  war,  the 
stream,  though  swelling  in  volume,  has  changed  both  its 
source  and  the  impelling  cause  of  its  flow.  The  early 
settlers  came  from  stock  which  had  made  notable  contri- 
butions to  civilization.  They  were  drawn  by  a  desire 
from  within  to  carve  out  great  names  and  fortunes.  And 
they  have  not  disgraced  their  kin  across  the  seas. 

This  tide  has  ebbed,  and  has  been  succeeded  by  a 
greater.  Fifteen  million  foreign-born  now  live  within 
the  boundaries  of  the  nation,  though  nearly  half  have 
never  sought  its  citizenship.  They  come  in  increasing 
numbers  from  peoples  who  have  impressed  modern  civil- 
ization but  lightly.  They  come,  not  so  much  from  inborn 
ambition  of  their  own,  but  because  attracted  by  the  in- 
ducements of  those  who  would  exploit  them  for  their 
own  convenience.  "Whether  any  considerable  part  oP 
these  people  are  genetically  undesirable,  whether  real 
capacity  will  be  discovered  under  the  new  environment, 
no  one  can  say.  Time  alone  will  tell.  But  there  is  a 
thought  in  this  connection  that  cannot  be  emphasized  too 
strongly  or  too  often.  To  make  this  a  united  nation, 
there  must  be  an  enormous  amount  of  open  racial  inter- 
mixture.    The  publicist  and  sociologist  should  realize 


INTERMINGLING  OF  RACES  265 

that  if  they  do  not  give  their  children  in  marriage  with 
the  immigrant,  they  must  with  the  immigrant's  children. 
Invidious  comparisons  are,  therefore,  unnecessary ;  ques- 
tions of  what  this  or  that  race  has  done  or  may  do  need 
not  be  settled.  It  is  quite  within  the  province,  it  is  indeed 
the  duty  of  the  native  citizen,  to  require  a  pause  in  this 
mad  rush  for  mere  population,  until  there  is  a  diffusion 
of  education  and  a  healthy  growth  of  a  nationalistic 
spirit.  By  the  time  this  has  been  accomplished,  the  result 
of  the  previous  policy  of  the  Open  Door  can  be  estimated 
more  justly,  and  any  necessary  adjustments  made  ^vith 
better  regard  for  the  good  of  all  the  people. 


LITEEATURE  ^ 

1  Allen_,  C.  E.  :  a  Chromosome  Difference  Correlated  with  Sex  Dif- 

ferences.   Science,  N.  S.,  1917,  xlvi,  466,  467. 

2  Arner^  G.  B.  L.  :  Consanguineous  Man^iages  in  the  American  Popu- 

lation.    Studies  in  Hist.,  Econ.  and  Pub.  Law,  1909,  xxxi.  No.  3. 

3  Beal^  W.  J. :  Eeports,  Michigan  Board  of  AgTiculture,  1876,  1877, 

1881  and  1882. 

4  Bell^  a.  G.  :  Memoir  upon  the  Formation  of  a  Deaf  Variety  of  the 

Human  Race.    Mem.  Nat.  Acad.  Sci.,  1884,  pp.  86. 

5  BemisS;  S.  M.  :  Report  on  Influence  of  Marriages  of  Consanguinity 

upon  Offspring.     Trans.  Amer.  Med.  Assn.,  1858,  xi,  321—425. 

6  Berthollet^  S.  :  Phenomenes  de  I'acte  mysterieux  de  la  fecondation. 

Mem.  Soc.  Linneenne  de  Paris,  1827,  i,  81-83. 
■^  Blyth^  E.  :  On  the  Physiological  Distinctions  between  Man  and  all 

other  Animals.    Mag.  Nat.  His.,  N.  S.,  1837,  i,  1-9,  77-85,  131-141. 
s  BoNHOTE,  J.  L. :  Vigour  and  Heredity.     London,  1915,  pp.  263. 
®  BouDiN^  M. :  Dangers  des  unions  consanguines  et  necessite  des  croise- 

ments  dans  I'espece  humaine  et  parmi  les  animaux.    Ann.  d'Hygiene 

pub.  et  de  Med.  legale,  1862,  xviii,  5-82. 

10  Bridges,  C.  B.  :   Non-disjunction  as  a  Proof  of  the  Chromosome 

Theory  of  Heredity.     Genetics,  1916,  i,  1-51,  107-163. 

11  Bridges,  C.  B.:  Deficiency.    Genetics,  1917,  ii,  445-465. 
12BRITT0N,  E.,  G.:  A  Hybrid  Moss.  Plant  World,  1898,  i,  138. 

13  Bruce,  A.  B.  :  A  Mendelian  Theory  of  Heredity  and  the  Augmenta- 

tion of  Vigor.    Science,  N.  S.,  1910,  xxxii,  627,  628. 

14  Bruce,  A.  B. :  Inbreeding.    Jour.  Gen.,  1917,  vi,  195-200. 

15  BuRGEOis,  A. :    Quelle  est  I'influence  des  mariages  consangnines  sur 

les  generations?    Theses  L'Ecole  de  Bled.,  1859,  ii.  No.  91. 

16  Carriej?.,  L.  :   The  Immediate  Effect  of  Crossing  Strains  of  Corn. 

Virginia  Agr.  Exp.  Sta.  Bull.  202,  1911,  pp.  11. 

a  This  list  of  literature  makes  no  pretension  of  citing  other  than  a  few 
of  the  most  important  books  and  papers  on  inbreeding  published  in  pre- 
Mendelian  days.  Those  interested  in  the  subject  from  the  standpoint  of 
marriages  of  near  kin  can  obtain  access  to  the  literature  by  following  up  the 
citations  of  Huth  and  of  Westermarck.  Tlie  real  development  of  the  subject 
has  come  from  the  investigations  on  heredity  completed  since  the  year  1900. 
Since  it  is  impracticable  and  unnecessary  to  cite*  all  the  genetic  work  of 
this  period,  only  those  titles  which  are  in  some  way  directly  connected  with 
the  subjects  discussed,  have  been  given. 

266 


LITERATURE  267 

17  Castle^  W.  E.:  The  Early  Embryology  of  Ciotia  intestinalis  Flem- 

ming  (L.).  Mus.  Com.  Zool.  Bull.  27, 1890,  201-280. 
IS  Castle^  W.  E.  :    Genetics  and  Eugenics.    Cambridge,  1910,  pp.  353, 
1^  Castle,  W.  E.,  and  Little,  C.  C:  On  a  Modified  Mendelian  Ratio 
Among  Yellow  Mice.    Science,  N.  S.,  1910,  xxxii,  808-870. 

20  Castle,  W.  E.,  and  Wright,  S.:  Studies  of  Inheritance  in  Guinea- 

pigs  and  Rabbits.     Carnegie  Inst.   Pub.  211,   Washington,  1910, 
pp.  192. 

21  Castle,  W.  E.,  Carpenter,  F.  W.  et  ah:  The  Effects  of  Inbreeding, 

Cross-breeding,  and  Selection  upon  the  Fertility  and  Vaiiability 
of  Drosophila.    Proc.  Amer.  Acad.  Arts  and  Sci.,  1900,  xli,  731-780. 

22  Cattell,  J.  M. :  A  Statistical  Study  of  American  Men  of  Science. 

Scie7ice,  N.  S.,  1900,  xxiv,  058-005,  099-707,  732-742. 

23  Caullery,  M.  :  Les  problemes  de  la  sexualite.    Paris,  1917,  pp.  332. 
2^  Chamberlain,  H.  S.:  The  Foundations  of  the  Nineteenth  Century. 

Trans.  J.  Lees.    2  vol.    New  York,  1910,  pp.  578-580. 

25  Chapeaurouge^  A.  de :  Einiges  liber  Inzucht  und  ihre  Leistung  auf 

verschiedenen  Zuchtgebieten.    Hamburg,  1909. 

26  Collins,  G.  N.  :  Increased  Yields  of  Corn  from  Hybrid  Seed.    Year- 

book U.  S.  Dept.  AgT.,  1910,  319-328. 

27  Collins,  G.  N.  :  The  Value  of  First  Generation  Hybrids  in  Corn. 

Bull.  191,  Bur.  Plant  Ind.,  U.  S.  Dept.  Agr.,  1910,  pp.  45. 

28  Collins,  G.  N.,  and  Kempton_,  J.  H. :  Effects  of  Cross-pollination  on 

the  Size  of  Seed  in  Maize.    Cir.  124  U.  S.  Dept.  Agr.  1913,  9-15. 

29  Collins,  G.  N.  :  A  More  Accurate  Method  of  Comparing  First  Gen- 

eration Hybrids  with  Their  Parents.     Jour.  Agr.  Res.,  1914,  iii, 
85-91. 

30  Collins,  G.  N.  :  Maize :  Its  Origin  and  Relationships.    Notes  of  the 

123d  Regular  Meeting  Bot.  Soc.  Wash.,  Jour.  Wash.  Acad.  Sci., 
1918,  viii,  42,  43. 

31  Cook,  0.  F. ;  The  Superiority  of  Line  Breeding  Over  Narrow  Breed- 

ing.    Bull.  140,  U.  S.  Dept.  Agr.,  Bur.  Plant  Ind.,  1909,  pp.  45. 

32  Coulter,  J.  M. :  The  Evolution  of  Sex  in  Plants.     Chicago,  1914, 

pp.  140. 

33  Cramer,  P.  J.  S. :  Kritische  Uebei^icht  der  bekannten  Fiille  von  Knos- 

penvariation.    Haarlem,  1907,  pp.  474. 

34  Crampe,   H.  :    Zuchtversuehe  niit   zahmen    Wanderratten.     Landw. 

Jahrh.,  1883,  xii,  389-458. 

35  Cull,  S.  W.  :  Rejuvenescence  as  the  Result  of  Conjugation.    Jour. 

Exp.  Zool.,  1907,  iv,  85-89. 

36  Daffner^  F.  :  Das  Wachstmn  des  Menschen.    Leipzig,  1902,  pp.  475. 


268         INBREEDING  AND  OUTBREEDING 

37  Danielson,  F.  H.,  and  Davenport,  C.  B.  :  The  Hill  Folk.    Mem.  1, 

Eugenics  Record  Offitce,  Cold  Spring  Harbor,  1912,  pp.  56. 

38  Darwin,  C.  :  The  Variation  of  Animals  and  Plant*  Under  Domesti- 

cation.  2ud  Ed.,  London,  1875,  2  vols.,  pp.  461-478. 

39  Darwin,  C.  :   The  Effects  of  Cross-  and   Self -Fertilization  in  the 

Vegetable  Kingdom.    London,  1876,  pp.  482. 

40  Davenport,  C.  B.  :  Heredity  in  Relation  to  Eugenics.    New  York, 

1911,  pp.  298. 

41  Da\^nport,    C.   B.  :     Inheritance   of    Stature.      Genetics,   1917,   ii, 

313-389. 
4  2  Davenport,  C.  B.,  and  Davenport,  G.  :  Heredity  of  Eye-color  in 

Man.    Science,  N.  S.,  1907,  xxvi,  589-592. 
4  3  Davenport,  C.  B.,  and  Davenport,  G.  :  Heredity  of  Hair-form  in 

Man.    Amer.  Nat.,  1908,  xlii,  341-349. 

44  Davenport,  C.  B.,  and  Davenport,  G.  :  Heredity  of  Hair-color  in 

Man.    Amer.  Nat.,  1909,  xliii,  193-211. 

45  Davenport,  C.  B.,  and  Davenport,  G.  :  Heredity  of  Skin-pigmenta- 

tion in  Man.    Amer.  Nat.,  1910,  xliv,  641-672;  705-731. 

46  Deniker,  J.:  The  Races  of  Man.    New  York,  1906,  pp.  611. 

47  Detlefsen,  J.  A. :  Genetic  Studies  on  a  Cavy  Species  Cross.     Car- 

negie Pub.  205,  Washington,  1914,  pp.  134. 

48  DuGDALE,  R.  L. :  The  Jukes.    New  York,  1877,  4th  Ed.,  1910,  pp.  121. 

49  DtJsiNG,  K. :  Die  Factoren  welehe  die  Sexualitat  entscheiden.    Jena, 

1883.     Inaug.  Dissertation. 

50  East,  E.  M.  :  Inbreeding  in  Com.    Connecticut  AgT.  Exp.  Sta.  Rpt. 

for  1907,  1908,  419-428. 

51  East,  E.  M.  :  A  Study  of  the  Factors  Influencing  the  Improvement 

of  the  Potato.     Illinois  Agr.  Exp.  Sta.  Bull.  127,  1908,  375-456. 

52  East,  E.  M.  :  The  Distinction  Between  Development  and  Heredity 

in  Inbreeding.    Amer.  Nat.,  1909,  xliii,  173-181. 

53  East,  E.   M.  ;   An  Interpretation   of   Sterility   in   Certain   Plants. 

Proc.  Amer.  Phil.  Soc,  1915,  liv,  70-72. 

54  East,  E.  M.  :  Studies  on  Size  Inheritance  in  Nicotiana.     Genetics, 

1916,  i,  164-176. 

55  East,  E.  M.  :   The  Bearing  of  Some  General  Biological  Facts  on 

Bud-variation.    Amer.  Nat.,  1917,  li,  129-143. 

56  East,  E.  M.  :  Hidden  Feeble-miiidedness.     Jour.  Her.,  1917,  viii, 

215-217. 

57  East,  E.  M.  :  The  Role  of  Reproduction  in  Evolution.    Amer.  Nat., 

1918,  lii,  273-289. 

58  East,  E.  M.,  and  Hayes,  H.  K.  :  Inheritance  in  Maize.    Conn.  Agr. 

Exp.  Sta.  Bull.  167,  1911,  pp.  141. 


LITERATURE  269 

59  East^  E.  M.,  and  Hayes^  H.  K.  :  Heterozygosis  in  Evolution  and  in 

Plant  Breeding.     Bull.  243,  U.   S.  Dept.  Agr.,  Bur.  Plant  Ind., 

1912,  pp.  58. 

60  Ellis,  H.:  A  Study  of  British  Genius.    London,  1904,  pp.  300. 

61  Emerson,  R.  A. :  Inheritance  of  Sizes  and  Shapes  in  Plants.    Amer. 

Nat.,  1910,  xliv,  739-746. 

62  Emerson,   R.   A. :    The   Inheritance   of   Certain    Abnormalities   in 

Maize.    Amer.  Breed.  Assn.  Rpt.  1912,  viii,  385-399. 

63  Emerson,  R.  A.,  and  East,  E.  M.  :  The  Inheritance  of  Quantitative 

Characters  in  Maize.    Nebraska  Agr.  Exp.  S'ta.,  Research  Bull.  2, 

1913,  pp.  120. 

64  Enriques,  p.  :  La  conjugazione  e  il  differenziamento  nogli  Infusoria. 

Arch.  f.  Protistenkiinde,  1907,  ix,  195-296. 

65  Estabrook,  a.  H.,  and  Davenport,  C.  B.  :  The  Nam  Family.    Mem. 

2,  Eugenics  Record  Office.    Cold  Spring  Harbor,  1912,  pp.  85. 

66  Fabre-Domengue,    p.  :    Unions    consanguines    chez    les    colombins. 

Ulntermediare  des  Biol.,  1898,  i,  pp.  203. 

67  Fay,  E.  a.  :  Marriages  of  the  Deaf  in  America.    Washington,  1898, 

pp.  527. 
6S  Fischer,   E.  :   Die  Rehobother   Bastards  und  das   Bastardierungs- 

problem  beim  Menschen.     Jena.     Review.     Jour.  Her.,  1914,  v, 

465-468. 
69  Fish,   H.   D.  :    On   the  Progi-essive   Increase   of  Homozygosis   in 

Brother-Sist-er  Matings.    Amer.  Nat.,  1914,  xlviii,  759-761. 
70FOCKE,  W.  0.:  Die  Pflanzen-Mischlinge.    Berlin,  1881,  pp.  569. 
71  Frazer,  J.  S. :  Totemism  and  Exogamy.    4  vol.,  London,  1910. 
7  2  Freud,  S.  :  Totem  and  Taboo.    Trans.  A.  A.  Brill.    New  York,  1918, 

pp.  265. 
73GALTON,  F.:  Hereditary  Genius.     2nd  Ed.,  London,  1892,  pp.  379. 
7*  Gartner,  C.  F.  :  Versuche  nnd  Beobachtungen  liber  die  Bastarder- 

zeugung  im  Pflanzenreich.    Stuttgart,  1849,  pp.  790. 

75  Gay,  C.  W.:  The  Breeds  of  Livestock.    New  York,  1916,  pp.  483. 

76  Gentry,  N.  W.  :  Inbreeding  Berkshires.    Amer.  Breed.  Assn.  Ann. 

Rpt.,  1905,  i,  168-171. 

77  Gernert,  W.  B.:  Aphis  Immunity  of  Teosinte-Com  Hybrids.    iS'ct- 

ence,  N.  S.,  1917,  xlvi,  390-392. 

78  Gerschleu,  M.  W.  :  Ueber  alternative  Vererbiing  bei  Kreuzung  von 

Cyprinodontiden-Gattungen.     Zeitschr.   f.  ind.   Ahst.  u.    Vererh., 

1914,  xii,  73-96. 

79  GOBINEAU,  Le  Compte  de :  Essai  sur  I'inegalite  des  races  humaines. 

2  vol.    Paris,  1884,  pp.  561-566. 

80  GODDARD,  H.  H. :  The  Kallikak  Family.    New  York,  1913,  pp.  121. 


270         INBREEDING  AND  OUTBREEDING 

81  GODDARD^  H.  H. :  Feeble-mindedness :  Its  Causes  and  Consequences. 

New  York,  1914,  pp.  599. 

82  GoLDSCHMiDT,  R.  I  Zuclitversuche  mit  Enten,  I.    Ztschr.  f.  ind.  Ahst. 

u.  Vererb.,  1913,  ix,  161-191. 

83  GouRDON,  J.:  Consangiiinite  chez  les  animaux  domestiques.     Ann. 

d; Hygiene  inib.  et  de  Med.  legale,  1862,  xviii,  463,  464. 

84  Grant,  M,  :  The  Passing  of  the  Great  Race.    2nd  Ed.    New  York, 

1918,  pp.  295. 

85  Gravatt,  F.  :  A  Radish-cabbage  Hybrid.     Jour.  Her.,  1914,  v,  269- 

272. 

86  GuAiTA,  G.  von :  Versuche  mit  Kreuzungen  von  verschiedenen  Rassen 

der  Hausmaus.     Ber.  d.  Naturforsch.   Gesell.  zu  Freiburg,  1898, 
X,  317-332. 

87  GuAiTA^  G.  von :  Zweite  Mittheilung  liber  Versuche  mit  Kreuzungen 

von  verschiedenen  Hausmausrassen.     Ber.  d.  Naturforsch.  Gesell. 
zu  Freiburg,  1900,  xi,  131-143. 

88  Haddon^  a.  C.  :  Races  of  Man.    London,  1909,  pp.  126. 

89  Hammond^  J. :  On  Some  Factors  Controlling  Fertility  in  Domestic 

Animals.     Jour.  Agr.  Sci.,  1914,  vi,  263-277. 
so  Hardy,  G.  H.  :  Mendelian  Proportions  in  a  Mixed  Population.    Sci- 
ence, N.  S.,  1908,  xxviii,  49,  50. 

91  Hartley,  C.  P.,  et  al.:  Cross-breeding  Corn.    Bull.  218,  U.  S.  Dept. 

Agr.,  Bur.  Plant  Ind.,  1912,  pp.  72. 

92  Hayes,   H.   K.  :    Com   Improvement   in   Connecticut.     Connecticut 

Agr.  Exp.  Sta.,  Rpt.  for  1913,  1914,  353-384. 

93  Hayes,  H.  K.,  and  East,  E.  M.  :  Improve-ment  in  Corn.    Connecticut 

Agr.  Exp.  Sta.  Bull.  168,  1911,  pp.  21. 

94  Hayes,  H.  K.,  and  East,  E.  M.  :  Further  Experiments  on  Inheri- 

tance in  Maize.    Connecticut  Agr.  Exp.  Sta.  Bull.  188,  1915,  pp.  31. 

95  Hayes,  H.  K.,  and  Jones,  D.  F.  :  The  Effects  of  Cross-  and  Self- 

fertilization  on  Tomatoes.     Connecticut  Agr.  Exp.  Sta.  Rpt.  for 
1916,  1917,  305-318. 

96  Herbert,  W.  :  Amaryllidaceas.     London,  1837,  pp.  428. 

97  HuTH,  A.  H. :  The  Marriage  of  Near  Kin.    London,  1875,  pp.  359. 

98  Hyde,  R.  H.  :   Fertility  and  Sterility  in  Drosophila  ampelophila. 

Jour.  Exp.  Zool.,  1914,  xvii,  141-171,  173-212. 
09  Jacoby,  p.  :  Etudes  sur  la  selection  chez  rhomme.    2nd  Ed.,  Paris, 

1904. 
i<^o  Janssens,  F.  a.:  La  theorie  de  la  ehiasmatypie.    La  Cellule,  1909, 

XXV,  389-414. 


LITERATURE  271 

101  Jennings,  H.  S.:  Heredity,  Variation  and  Evolution  in  Protozoa. 

II.  Heredity  and  Variation  of  Size  and  Form  in  Paramecium,  with 

Studies  of  Growth,  Environmental  Action,  and   Selection.     Vroc. 

Amer.  Phil.  Soc,  1908,  xlvii,  393-546. 
10  2  Jennings,   H.   S.:   Production   of  Pure   Homozygotic    Or^^anisras 

from  Heterozygotes  by  Self-Fertilization.    Amer.  Nat.,  1912,  xlvi, 

487-491. 

103  Jennings,  H.  S.  :  The  Effect  of  Conjugation  in  Paramecium.    Jour. 

Exp.  Zool.,  1913,  xiv,  279-391. 

104  Jennings,  H.  S.  :  Formula}  for  the  Results  of  Inbreeding.     Amer. 

Nat.,  1914,  xlviii,  693-696. 

105  Jennings,  H.  S.  :  The  Numerical  Results  of  Diverse  Systems  of 

Breeding.    Genetics,  1916,  i,  53-89. 
loe  Jennings,  H.  S.  :   The  Numerical  Results  of  Diverse  Systems  of 
Breeding,  with  Respect  to  Two  Pairs  of  Characters,  Linked  or  In- 
dependent, with  Special  Relation  to  the  Effects  of  Linkage.     Gen- 
etics, 1917,  ii,  97-154. 

107  Jennings,  H.  S.:  Heredity,  Variation  and  the  Results  of  Selection 

in  the  Uniparental  Reproduction  of  Bifflugia  corona.     Genetics, 
1916,  i,  407-534. 

108  Jennings,  H.  S.,  and  Lashley,  K.  S.  :  Biparental  Inheritance  and 

the  Question  of  Sexuality  in  Paramecium.    Jour.  Exp.  Zool.,  1913, 
xiv,  393-466. 

109  Johannsen,  W.  :  Ueber  Erblichkeit  in  Populationen  und  in  reinen 

Linien.    Jena,  1903,  pp.  68. 

110  Johannsen,  W.  :   Elemente  der  exakten  Erbliehkeitslehre.     Jena, 

1909,  pp.  515. 

111  Jones,  D.  F.  :  Dominance  of  Linked  Factors  as  a  Means  of  Account- 

ing for  Heterosis.    Proc.  Nat.  Acad.  Sci.,  1917,  iii,  310-312.    Also 
Genetics,  1917,  ii,  466-479. 

112  Jones,  D.  F.  :  Bearing  of  Heterosis  upon  Double  Fertilization.    Bot. 

Gaz.,  1918,  Ixv,  324-333. 
112  Jones,  D.  F.  :    Tlie  Effects  of  Inbreeding  and  Cross-liroediiig  upon 

Development.    Conn.  AgT.  Exp.  Sta.  Bull.  207,  1918,  pp.  100. 
114  Jones,  D.  F.,  and  Hayes,  H.  K.  :  Increasing  the  Yield  of  Corn  by 

Crossing.    Connecticut  Agr.  Exp.  Sta.  Rpt.  for  1916, 1917,  323-347. 
iisjoiiGER,  J.:   Die   Familie  Zero.     Arch.  f.   Bass.   u.   Gesellschafts- 

biologie,  1905,  ii,  494-559. 
116  Keeble,  F.,  and  Pellew,  C.  :  The  Mode  of  Inheritance  of  Stature 

and  of  Time  of  Flowering  in  Peas  {Pisum  sativum).    Jour.  Gen., 

1910,  i,  47-56. 


272         INBEEEDING  AND  OUTBREEDING 

117  King,  H.  D.:  On  the  Normal  Sex  Ratio  and  the  Size  of  the  Litter 

in  the  Albino  Rat  {Mus  norvegicus  aThinus).  Anat.  Bee,  1915, 
ix,  403-419. 

118  King,  H.  D.:  The  Relation  of  Age  to  Fertility  in  the  Rat.    Anat, 

Bee,  1916,  xi,  269-287. 

1 19  King,  H.  D.  :  Studies  on  Inbreeding.    I.  The  Effects  of  Inbreeding 

on  the  Growth  and  Variability  in  Body  Weight  of  the  Albino  Rat. 
Jour.  Exp.  Zool.,  1918,  xxvi,  1-54. 

120  King,  H.  D.  :  Studies  on  Inbreeding.    II.  The  Effects  of  Inbreeding 

on  the  Fei-tility  and  on  the  Constitutional  Vigor  of  the  Albino  Rat. 
Jour.  Exp.  Zool.,  1918,  xxvi,  55-98. 

121  King,  H.  D.:  Studies  on  Inbreeding.    III.  The  Effects  of  Inbreed- 

ing with  Selection,  on  the  Sex  Ratio  of  the  Albino  Rat.  Jour. 
Exp.  Zool.,  1918,  xxvii,  1-35. 

122  Knight,  T.  A. :  An  Account  of  Some  Experiments  on  the  Fecunda- 

tion of  Vegetables.     Phil.  Trans.  Boy.  Soc,  Lon.,  1799,  Ixxxix, 
195-204. 
123 Knight,  T.  A.;  Physiological  and  Horticultural  Papers.    London, 
1841,  pp.  389. 

124  Knuth,   p.  :  Handbuch   der   Bliitenbiologie.     Leipzig,   1898-1905. 

3  vol. 

125  KbLREUTER,  J.  G. :  Dritte  Fortsetzung  der  vorlaufigen  Nachricht 

von  einigen  das  Geschleoht  der  Pflanzen  betreffenden  Versuchen 
und  Beobactungen.  Leipzig,  1766,  pp.  156.  (Reprinted  in  Ost- 
wald's  Klassiker  der  exakten  Wissenschaften,  No.  41,  Leipzig, 
1893.) 

126  Kraemer,  H.  :  Ueber  die  ungiinstigen  Wirkungen  naher  Inzucht. 

Mitt.  d.  deut.  landw.  GeseU.,  6  and  13,  1913.  Trans.  Jour.  Her., 
1913,  v,  226-234. 

127  Lecoq,  H.  :  De  la  fecondation  naturelle  et  artificielle  de  vegetaux  et 

de  rhybridation.    Pai-is,  1845,  pp.  287. 
128LINDLEY,  J.:  The  Theory  of  Horticulture.     2nd  Ed.,  New  York, 
1852,  pp.  364. 

129  LoEB,  J. :  The  Organism  as  a  Whole.     From  a  Physico-chemical 

Viewpoint.    New  York,  1916,  pp.  379. 

130  Marchal,  el.,  and  Marchal  em.:  Aposporie  et  sexuaUte  chez  les 

Mousses.  I,  II,  III.  Bull.  Acad.  Boy.  Belg.,  CI.  S'ci.,  1907,  765- 
789;  1909,  1249-1288;  1911,  750-778. 

131  McCluer,  G.  W.:  Com  Crossing.    Illinois  Agr.  Exp.  Sta.  Bull.  21, 

1892,  82-101. 

132  McCuLLocH,  0.  C. !     The  Tribe  of  Ishmael:     A  Study  in  Social 
Degradation.    Proc.  15th  Natl.  Conf.  Char,  and  Cor.,  1888. 


LITERATURE  273 

138  MacNamara,  N.  C:  Origin  and  Cliaracter  of  the  British  People. 

London,  1900,  pp.  242. 
134]VIarshall,  F.  H.  a.:  The  Physiolo^-  of  Reproduction.     London, 

1910,  pp.  706. 

135  jyi^BTjj^^  R, :  Lehrbuch  der  Antliropologie  in  systematischer  Dar- 

stellung.    Jena,  1914,  pp.  1181. 

136  ]yxAUPAS_,  E. :  Reeherches  experimentales  sur  la  multiplication  des 

infusoires  cilies.  Arch.  d.  Zool.  Exp.  et  Gen.,  'II,  LSS9,  vi,  1(15-277. 

137  Maupas,    E.:    La    rajeunissement    karyogamique    chez    les    cili6s. 

Arch.  d.  Zool  Exp.  et  Gen.,  II,  1889,  vii,  149-517. 

138  Mauz,  E.  :  In  Correspondenzblatt  des  Wiirttemburgischen  Landw. 

Ver.  1825. 

139  Mendel,  G.  J.:  Versnche  iiber  Pflanzen-Hybriden.     Verh.  Naturf. 

Ver.  in  Briinn,  1865.     Trans,  in  Castle's  Genetics  and  Eugenics, 

Cambridge,  1916,  pp.  281-321. 
i^oMerz,  J.  T. :  A  Histoi-y  of  European  Thought  in  the  Nineteenth 

Century.     3rd  Ed.,  3  vol.,  London,  1907. 
141Metz^   C.   W.  :   The  Linkage   of   Eight   Sex-linked   Characters   in 

Drosophila  virilis.     Genetics^  1918,  107-134. 

142  MiDDLETON,  A.  R.  I  Heritable  Variations  and  the  Results  of  Selec- 

tion in  the  Fission  Rate  of  Stylonychia  pustulata.     Jour.  Exp. 
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143  Mitchell^  A. :  Blood-relationship  in  Marriage,  Considered  in  Its 

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Morph.,  1911,  xxii,  123-154. 

145  Montgomery,  E.  G.  :  Preliminaiy  Report  on  Effect  of  Close  and 

Broad  Breeding  on  Productiveness  in  Maize.    Nebraska  Agr.  Exp. 
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146  Morgan,  T.  H.  :   Sex-limited  Inheritance  in  Dix)sophila.     Science, 

N.  S.,  1910,  xxxii,  120-122. 

147  Morgan,  T.  H.  :  Chromosomes  and  Associative  Inheritance.     Sci- 

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148  Morgan,  T.  H.  :  An  Attempt  to  Analyze  the  Constitution  of  the 

Chromosomes  on  the  Basis  of  Sex-limited  Inheritance  in  Droso- 
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149  Morgan,  T.  H.  ;  Heredity  and  Sex.    New  York,  1913,  pp.  282. 

150  Morgan,   T.   H.,   and  Bridges,   C.   B.  :    Sox-linked  Inheritanoe  in 

Drosophila.     Carnegie  Ins.  Pub.  237,  Washington,  1916,  pp.  87. 
18 


274         INBREEDING  AND  OUTBREEDING 

151  Morgan,  T.  H.,  Sturtevant,  A.  H.,  Muller,  H.  J.,  and  Bridges, 

C.  B. :  The  Meclianism  of  Mendelian  Heredity.    New  York,  1915, 
pp.  262. 

152  Morrow,  G.  E.,  and  Gardner,  F.  D.  :  Field  Experiments  witli  Com. 

Illinois  Agr.  Exp.  Sta.  Bull.  25,  1893,  173-203. 

153  Morrow,  G.  E.,  and  Gardner,  F.  D.  :  Experiments  with  Corn.    Illi- 

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1 55  MuLLER,  H.  J. :  An  (Enothera-like  Case  in  Drosophila.    Proc.  Nat. 

Acad.  ScL,  1917,  iii,  619-626. 

156  MuLLER,  H.  J. :  Genetic  Variability,  Twin  Hybrids  and  Constant 

Hybrids,  in  a  Case  of  Balanced  Lethal  Factors.     Genetics,  1918, 
iii,  422-499. 

157  MtJLLER,  H. :  Die  Befruchtung  der  Blumen  durch  Insekten  und  die 

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158  MuMFORD,   F.   B.  I   The  Breeding  of  Animals.     New  York,   1917, 

pp.  303. 

159  Naudin,  C.  :  NouveUes  recherches  sur  I'hj^bridite  dans  les  vegetaux. 

Nouv.  Arch.  du.  Mus.  d'Hist.  Nat.  de  Paris,  1865,  i,  25-174. 

160  Bearing,  S.:  Geographical  Distribution  of  American  Genius.    Pop. 

Sci.  3Ion.,  1914,  Ixxxv,  189-199. 

161  Nearing,  S.  :  The  Younger  Generation  of  American  Genius.     Sci. 

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162  Nemec,  B.  :  Das  Problem  der  Befruchtmigsvorgange.    Berlin,  1910, 

pp.  532. 

163  Odin,  A. :   Genese   des  grandes  hommes  gens   de   lettres   fran^ais 

modernes.    2  vol.,  Paris,  1895. 
16-1  Parker,  G.  H.,  and  Bullard,  C.  :  On  the  Size  of  Littei^  and  tJie 

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Fowl.    Jour.  Exp.  Zool.,  1912,  xiii,  153-268. 

166  Pearl,  R.  :  On  the  Con-elation  Between  the  Number  of  Mammas  of 

the  Dam  and  Size  of  Litter  in  Mammals.    I.    Interracial  Correla- 
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167  Pearl,  R.  :  On  the  Correlation  Between  the  Number  of  Mammge  of 

the  Dam  and  Size  of  Litter  in  IVIammals.    II.    Intraracial  Correla- 
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168  Pearl,  R.  :  A  Contribution  Towards  an  Analysis  of  the  Problem  of 

Inbreeding.     Amer.  Nat.,  1913,  xlvii,  577-614. 


LITERATUEE  275 

169  Pearl,  R.,  and  Miner,  J.  R. :  Tables  for  Calculating  CoefTicicnts  of 
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a  Correction  and  Extension  of  Previous  Conclusions.   Amer.  Nat 
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171  Pearl,  R.  :  On  a  General  Formula  for  the  Constitution  of  the  N'ih 

Generation  of  a  Mendelian  Population  in  which  all  Matings  are  of 
Brother  X  Sister.    Amer.  Nat.,  1914,  xlviii,  491-494. 

172  Pearl,  R.  :    Inbreeding  and  Relationship  Coefhcients.     Amer.  Nat., 

1914,  xlviii,  513-523. 

178  Pearl,  R.:   Modes  of   Research   in   Genetics.     New   York,    1915, 

pp.  182. 

174  Pearl,   R.  :  Some   Further   Considerations   Regarding   Coasin    and 

Related  Kinds  of  Mating.    Amer.  Nat.,  1915,  xlix,  570-575. 

175  Pearl,  R.  :  Some  ruither  Considerations  Regarding  the  Measure- 

ment and  Numerical  Expression  of  Degrees  of  Kinship.     Amer. 
Nat.,  1917,  li,  545-559. 

176  Pearl,  R.  :  A  Single  Numerical  Measure  of  the  Total  Amount  of 

Inbreeding.    Amer.  Nat.,  1917,  li,  636-639. 

177  Pearson,  K.  :  On  a  Generalized  Theory  of  Alternative  Inheritance, 

with  Special  References  to  Mendel's  Laws.    Phil.  Trans.  Roy.  Soc. 
(A),  1904,  cciii,  53-86. 
17«  Phillips^  J.   C. :   Size  Inheritance  in  Ducks.     Jour.  Exp.  Zool., 
1912,  xii,  369-380. 

179  Phillips,  J.  C:  A  Further  Study  of  Size  Inheritance  in  Ducks, 

with  Observations  on  the  Sex  Ratio  of  Hybrid  Birds.    Jour.  Exp. 

Zool.,  1914,  x^d,  131-148. 
160  Plate,  L.  :  Vererbungslehre.    Leipzig,  1913,  pp.  519. 
181  popENOE,  P.,  and  Johnson,  R.  H.  :  Applied  Eugenics.    New  York, 

1918,  pp.  459. 
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Poultry.    Jour.  Gen.,  1914,  iv,  23-39. 
183  Ripley,  W.  Z.:  The  Races  of  Europe.    New  York,  1899,  pp.  624. 
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185  Roberts,  H.  F.  :  First  Generation  Hybrids  of  American  and  Chi- 

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186  RoBBiNS,,  R.  B. :   Some  Applications  of  Mathematics  to  Breeding 

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187  RoBBiNS,  R.  B. :  Random  Mating  with  the  Exception  of  Sister  by 

Brother  Mating.    Genetics,  1918,  iii,  390-39G. 


276         INBREEDING  AND  OUTBREEDING 

188  RoMMELL^  G.  M. :  The  Fecundity  of  Poland-China  aiid  Duroc-Jersey 
Sows.     Cir.  95,  U.  S.  Dept.  Agr.,  Bur.  An.  Ind.,  1906,  pp.  12. 

ii89  RoMMELL^  G.  M. :  The  Inheritance  of  Size  of  Litter  in  Poland- 
China  Sows.     Amer.  Breed.  Assn.  Rpt.,  1907,  v,  201-208. 

190  RoMMELL^  G.  M.,  and  Phillips^  E.  F.  :  Inheritance  in  the  Female 

Line  of  Size  of  Litter  in  Poland-China  Sows.  Proc.  Amer.  Phdl. 
Soc,  1906,  xlv,  245-254. 

191  Sageret,  a.  :    Considerations  snr  la  production  des  hyb rides,  des 

variantes  et  des  varietes  en  general,  et  sur  celles  de  la  famille  de 
Cucurbitacees  en  particulier  Ann.  des  Sci.  Nat.,  1826,  viii,  294- 
314. 

192  Shamel^  a.  D.  :  The  Effects  of  Inbreeding  in  Plants.     Yearbook 

U.  S.  Dept.  Agr.,  Washing1)on,  1905,  pp.  377-392. 

193  Shull_,  G.  H.  :  The  Composition  of  a  Field  of  Maize.    Amer.  Breed. 

Assn.  Rpt.,  1908,  iv,  296-301. 

194  Shull_,  G.  H.  :  A  Pure  Line  Method  of  Com  Breeding.     Amer. 

Breed.  Assn.  Rpt.,  1909,  v,  51-59. 

195  Shull^  G.  H.  :  Hybridization  Methods  in  Corn  Breeding.     Amer. 

Breed.  Mag.,  1910,  i,  98-107. 

196  Shull,  G.  H.  :  The  Genotypes  of  Maize.     Amer.  Nat.,  1911,  xlv, 

234-252. 

197  Shull,  G.  H.  :  Duplicate  Genes  for  Capsule  Form  in  Bursa  hursa- 

pastoris.    Zeitschr.  f.  ind.  Ahst.  u.  Vererh.,  1914,  xii,  97-149. 

198  Shull,  a.  F.  :  The  Influence  of  Inbreeding  on  Vigor  in  Hydatina 

senta.    Biol.  Bull.,  1912,  xxiv,  1-13. 

199  Shull^  a.  F.  :  Studies  in  the  Life  Cycle  of  Hydatina  senta.    III. 

Jour.  Exp.  Zool.,  1912,  xii,  283-317. 

200  Strasburger^  E.  :    Versuche  mit  diocischen  Pflanzen  in  Riicksioht 

auf  Geschlechtsverteilung.    Biol.  Centralbl.,  1900,  xx,  657. 

201  Strasburger^  E.  :  Ueber  geschlechtbestimmende  Ursachen.     Jahrb. 

Wiss.  Bot.,  1910,  xlviii,  427-520. 

202  Sturtevant^  A.  H. :  The  Linear  Arrangement  of  Six  Sex-linked 

Factors  in  Drosophila,  as  Shown  by  Their  Mode  of  Association. 
Jour.  Exp.  Zool,  1913,  xiv,  43-59. 

203  Sturtevant,  A.  H. :  The  Behavior  of  the  Chromosomes  as  Studied 

through  Linkage.    Zeitschr.  f.  ind.  Ahstam.  u.  Vererh.,  1915,  xiii, 
234-287. 

204  Surface,  F.  M.  :  Fecundity  in  Swine.    Biometrika,  1909,  vi,  433-436. 

205  Toyama,  K.  :  Mendel's  Law  of  Heredity  as  Applied  to  Silkworm 

Crosses.    Biol.  Chi,  1906,  xxvi,  321-334. 

206  VoisiN,  A. :  Contribution  a  Fhistoire  des  manages  entre  oonsangnins. 

Compt.  rend.  Acad.  Sci.,  1865,  Ixv,  105-108. 


LITERATURE  277 

207  Warren,  H.   C.  :   Numerical  Effects  of  Natural   Selection   Acting 

Upon  Mendelian  Characters.     Genetics,  1917,  ii,  305-312. 

208  Weinstein,  a.  :  Coincidence  of  Crossing  Over  in  Drosophila  mel- 

anogaster  [ampelophila) .    Genetics,  1918,  iii,  135-159. 

209  Weismann,   a.    The  Evolution   Theory.     (Trans.   J.   A.   Thomson 

and  M.  R.  Thomson.)     London,  1904,  2  vol. 

210  Wellington,  R.  :  Influence  of  Crossing  in  Increasing  the  Yield  of 

the  Tomato.    New  York  Agr.  Exp.  Sta.  Bull.  34G,  1912,  57-76. 

21 1  Wentworth,   E.   N.  :    The   Segi^egation   of  Fecundity   Factors  in 

Drosophila.    Jour.  Gen.,  1913,  iii,  113-120. 

212  Wentworth,  E.  N.,  and  Aubel,  C.  E.  :  Inheritance  of  Fertility  in 

Swine.    Jour.  Agr.  Bes.,  1916,  v,  1145-1160. 

21 3  Wentworth,  E.  N.,  and  Remick,  B.  L.  :  Some  Breeding  Properties 

of  the  Generalized  Mendelian  Population.     Genetics,  1916,  i,  608- 
616. 

214  Westermarck,  E.  :  The  History  of  Human  Marriage.     3rd  Ed., 

London,  1903,  pp.  644. 

215  Wheeler,  W.  M.  :  The  Ants  of  the  Baltic  Amber.    Schrlft.  Phijsik- 

okonom.  GeseU.  Kordgsherg,  1914,  Iv,  pp.  142. 

216  Whitney,  D.  D.  :  Reinvigoration  Produced  by  Cross-fertilization  in 

Hydatina  senta.    Jour.  Exp.  Zool.,  1912,  xii,  337-362. 

217  Whitney,  D.  D.  :  "  Strains  "  in  Hydatina.    Biol.  Bull.,  1912,  xxii, 

205^218. 

218  Whitney,  D.  D.  :  Weak  Parthenogenetic  Races  of  Hydatina  senta 

Subjected  to  a  Varied  Environment.    Biol.  Bull.,  1912,  xxiii,  321- 

330. 

219  WiEGMANN,  A.  F. :  Ueber  die  Bastarderzeugung  im  Pllanzenreich. 

Braunschweig,  1828,  pp.  40. 

220  Withington,  C.  F.  :  Consanguineous  Marriagas :    Their  Effect  upon 

Offspring.    Mass.  Med.  Soc,  1885,  xiii,  453-484. 

221  Wolfe,  T.  K.;  Further  Evidence  of  tlie  Immediate  Effect  of  Cross- 

ing Varieties  of  Corn  on  the  Size  of  the  Seed  Produced.     Jour. 
Amer.  Soc.  Agr.,  1915,  vii,  265-272. 

222  Woodruff,   L.   L.:    Two    Thousand    Generations   of   Paramecium. 

Arch.  f.  Protistenkunde,  1911,  xxi,  263-266. 

223  Woods,  F.  A. :  Heredity  in  Royalty.    New  York,  1906,  pp.  312. 

224  Woods,   F.   A.:    The   Influence  of  Monarchs.     New   Y^ork,    1013, 

pp.  421.  T    T^     r 

225  Wright,  S.:    The  Effects  of  Inbreeding  on  Guinea-pigs.   T.  Dei>lino 

in  Vigor.  II.  Differentiation  among  Inbred  Families.  III.  Crosses 
between  Different  Highly  Inbred  Families.  (Doctor  Wright  kindly 
pemiitted  the  authors  to  read  these  valuable  unpublished  papers 
in  manuscript. ) 


PROPERTY  UBlUjir 

fj[.  C.  StaU  ColKS^ 


INDEX 


Achondroplasy,   230 

Adaptation,  for  cross-pollination,  34 

for  self-pollination,  30 
Africa,  247,  254,  257 

native  population  of,   253 
Agassiz,  236 
Agriculture,    18 
Algae,  204 
Allelomorphs,  55 
Allen,  45 

Alternation  of  generations,  29,  46 
Althcea,   144 
America,  247,  252 
Amoeba  in  division,   21 
Amorites,  261 
Amphimixis,  206 
Angles,  260 
Annelids,  21 
Anthropology,  18,  245 
Apomixis,  32 
Apple,  210 
Arabs,  261 
Armadillo,   42 

nine-banded,      identical      quadru- 
plets in,  44 
Arthropoda,  22 
Arthropods,  2 If.,  207 
Aryan,  258 

Celtic,  259 

Nordic,  259f. 

races,  256 

Teutonic,   258 
Asia,  247,  258 

culture  of,  256 
Assyrioides,   261 
Atavism,   166 
Ataxia,  Friedrich's,  231 
Australia,  247,  252 
Autogamy,  30 

Barley,   114,  210 
Basidiomycetes,  31 


Bateson,    105 
Bcal,   221 
Beans,  114 
Bees,    158 
Berthollet,    Ml 
BihoSy  frontalis,   192 

gaums,   192 

(jr  aniens,    102 
Birds,    158 

Bison   anicricnnns,    192 
Blossom's  Glorcne,  S5ff. 
Bos  taurus,  192 
Brachydactyly,  230 
Brassica  olcracea,  192 
Breckenridge,    235 
Bridges,  184 
Britain,    2o8f. 
British   Isles,   258 
Bronze  Age,  258 
Briinnow,  236 
Bryozoans,   23 
Budin,  227 
Buffalo,   192 

Cabbage,   192 

Caesar's  invasion  of  Britain,  259 

Calceolaria,   155 

Castle,  25,   111,   137,   158,   160,   188, 

236 
Cat,  211 
Catalpa,   150 
Cataract,  230 
CsittiiW,  233 
Cattle,  210,  213 
Caucasian,   255 
Caullery,  22 
Cavia,    103 

cutlcri,  160 

species,  hybrids,  192 
Celts,  258 
Cereals,  211 
Cephalic  index,  249 

279 


280 


INDEX 


Cliemotropism,    154 

China,   247 

Chinese,  254 

Chordates,  21 

Chromosomes,  36 

Ciona  intestinalis,  25 

Cirriped.es,  25,  34 

Cleft  palate,  230 

Coccinea,   144 

Coefficient   of   cross-relationship,    86 
of  heredity,   196,   199 
of  inbreeding,  81/f. 
of  relationship,   81,  84^. 

Coelenterates,  21 

Collins,  137,   153 

Coloration,   protective,    146 

Color-blindness,  47^. 

Complemental    males,    25 

Composite^,  32 

Conjugatce,  27 

Connate  seeds  of  maize,  133 

Consanguinity,  113,  139 

Corn,  varieties  of,   215 

Coulter,  26 

Cow,  192 

Cramer,  198 

Crampe,    101^. 

Cromwell,   258 

Crossing-over,  diagram  to  illustrate, 

64 
Cucumber,    221 
Cucumis,   144 
Cymotlioidce,   24 
Cytoplasm  of  egg,  200 

Dalton,  235 

Danes,  260 

Darwin,   13,  25,  32,  34,   101,   114^., 

137^.,  143,  146^.,  154,  164/.,  186, 

235 
Datura,    142,    144 
Davenport,   230/.,   233 
Delphino,    34 
Detlefsen,  103,  192 
Diabetes  insipidus,  230 
Dianthus,  116/.,  142,  144,  155 


Diatotnece,  27 

Dichogamy,  33 

Difflugia,   203 
coronata,  78 

Digitalis,    144,    155 

Dioecism,  22 

Disease,     susceptibility    and     resis- 
tance to,   134 

Dog,  210f. 

Dominance,    57,    72,    177^. 

Dominant     factors,     complementary 
action  of,  171 

Double  cross,  223 

Double  fertilization,  203. 

Draha,  155 

Drosophila,    179,    188,    198,   208 
melanogaster,  61,   111,   184 
sterility  in,  112 

Diising,  106 

Echinodermata,  22 
Echinoderms,   21 
Edwards,   235 
Ellis,  233 
Emerson,    183- 
Endogamy,    238 
Endosperm   fertilization,    153 
England,  257^.,  260/. 
Englishman,    250,   262 
Epilepsy,  231,  241 
Eschscholtzia,  117 
Europe,  247,  251/.,  257,  261 

leaders  of,  257 
European  culture,  256 
Evolution,  13 

inbreeding    and    outbreeding    in, 
195^. 
Exogamy,  13,  15,  201 

Factors,  stability  of,  76f. 
Faraday,  235 
Feeble-mindedness,  231 
Ferns,  29 

Fertilization,  diagram  to  illustrate, 
39 

double,   41 

in  embryo  sac  of  the  lily,  41 


INDEX 


281 


Fish,  92,  158 

Fitzhugh,    235 

Flat  worms,  21 

Focke,   141,   145 

Fossil   ants  in   amber   of  Oligocene 

period,  77 
France,  257f.,  260/=. 
Franklin,    235^. 
Franks,   260 
Frazer,    13 
Freeman,    157 
French,  257 
Frenchman,  250 
Frequency    distribution    of    corolla 

length  in  tobacco,  70 
Freud,  13 
Fucus,  26,  28 
Funaria,  46 

Galton,  50,  227,  233,  237 
Gamete  formation  in  dihybrid,  59,  63 
Gametogenesis,   38,   55 
Gametophyte,  29 
Gardner,  118 

Gartner,  141,  143,  145,  155 
Gaur,   192 
Gayal,  192 
Genetics,  50 

Genotype  hypothesis,  166 
Gentry,  213 

Germany,  257,  260,  262 
Germplasm,  mixture  of,  201 
Gernert,  155 
Gerschler,    158 
Geum,   144 
Goat,  212 
Goddard,  240,  243 
Goliath,  110    (fig.  28),  233 
Gonochorism,  22,  33 
Grape,  210 
Gravatt,  192 
Greece,    251 
Green  Algser,  26 
Guaita,  von,   101^. 
Guinea-pig,   188 
growing  curves  of,  160 


Guinea-pig,  inbreeding  experiments 
with,   110^. 

Haiti,  253 

Hammurabi,  code  of,  14 

Hare- lip,  230 

Hawkweed,  22 

Hayes,  89,   148,  168,  192 

Herbert,    141 

Heredity  coefficient,  196,  199 

Hermaphroditism,  22^.,  26,  30,  33, 

201 
Hero,    117 

Heterosis,  16,  96,  141,  144,  157,  172, 
202 

importance  of  in  sex  origin,  20i ff. 

manifestations  of,   150 

selective  effect  of,  154 
Heterozj^gosis,   121,   138 

degrees  of,  93 

similarity   of   effect   of,   with    en- 
vironment, 157 
Hittites,   261 
Homozygosis,    113 
Homozygosity,   134 

affected  by  linkage,  95 

attainment  of,  95 
Horse,  212 

origin  of,  2l0ff. 
Huntington's  chorea,  230 
Huth,  243 
Hybrid  vigor,  16,  88,  96,  141 

benefit  from,  219 

cause  of,   164 
Bydatina,  158 

senta,  ll2f. 
Hyde,  112,  158 
nyoscyamus,  155 

Iberian,  259 

race,   258 
Ichythyosis,  230 
Inbreeding,  curves  of,  84 

effect  of,  on  yield  and  height  of 
maize,    124^. 

effect  on  organisms,   137 


282 


INDEX 


Inbreeding,    experiments    with    ani- 
mals and  plants,  100 

index,  81 

intensity  of,  85 

mathematical    considerations    of, 
80 

problem,  phases  of,  81 

reduction  in  vigor  resulting  from, 
96 

and    outbreeding    in    plant    and 
animal  improvement,  210 
Infant  consultations,  227 
Infusoria,  78 

Inheritance,  Mendelian,   72 
Insanity,   231,   241 
Insects  in  Oligocene  amber,   197 
Ipomea,    115,    117 
Ireland,  257^. 
Irish,   260 
Isopods,  24 

Japan,  247 

Japanese,  254f. 

Jennings,   78,   92,   95,  97,  202/. 

Jew,   261/. 

English,    262 

German,    261 

Spanish,  261 
Johannsen,  78,  166 
Jukes,  237^.,  260,  262 
Jutes,  259 

Keeble,  170,  172 

Kempton,  153 

Kerner,   34 

King,   101/f.,    105,   107,   160,   188 

King  Melia  Rioter  14th,  85^. 

Knight,   114,  141^. 

Knight-Darwin    law,    33 

Knuth,  34 

Kolreuter,  141/.,   144,  154 

Lang,    13 
Lavatera,   144 
Learning   strains,    124 
Lecoq,   141 


Lee,  235 

LeguminoscB,    114 
Lethal  factors,  179 
Linaria,   144 
Lincoln,  235/. 
Lindley,   143 
Liverworts,  29,  45 
Lobelia,  144 
Loeb,    200 
Lychnis,   144 
Lycium,    144 
Liiffa,   144 

MacLennan,  13 

MacNamara,   257 

Maize,  connate  seeds  of,  133 

distribution    of     rows     of    grain 
of,    129 

fertility  of,  188^. 

growth  curves  of,  152 

inbred    strains    and    hybrids    of, 
150    (Fig.   31) 

inbred  strains  of,  130   (Fig.  29) 

number   of  nodes  of,    150 

segregation    of    ear    row    number 
of,    131,    132 

variety  crosses  of,  153 
Malthus,  247 
Malva,  144,  155 
Mammals,  crossing  of,    159^. 
JNIan,    inbreeding    and    outbreeding 
in,  226 

inter  fertility  of,  246 
Marchals,  46 

Marriage  of  near  relatives,  100 
Matings,  brother  and  sister,  97 

parent    and    offspring,    97 
Mauz,  141 

Mecca  of  politically  oppressed,  264 
Mechanism   of  heredity,   50 

of  reproduction,  36 
Mendel,  50,  118,  144/.,  165 
Mendelian   segregation,    88 
Mendelism,    51^. 

Mendel's  laws  of  inheritance,  55 
Merz,  233 


INDEX 


283 


Mice,    188 

Middleton,   78 

Miela,  227 

Milk  depots,  227 

Mimicry,   146 

Mimulus,    115,   117 

Mirabilis,    142 

Moenkhaus,    112,   158 

Molluscoids,    21 

Molluscs,  21 

Mongolian,  255,  257 

Monoecism,  33 

Morgan,  62,  198 

Morphology,    comparative,    18 

Morrow,    118 

Moss,  29,  46,  204 

Mule,    142,    219/. 

Miiller,   34 

Muller,  158 

Mumford,  214 

Myxomycetes,    26 

Nam,  237f.,  260,  262 
ISTaudin,  144 
Nearing,  233 
Negro,   252^. 
Nemathelminthes,  22 
Nematode,  21 
Nemec,  203 
Neolithic  period,  258 
New  Zealand,  247 
Nicotiana,   103,   139,    142,    144, 
155,    157,    192,    196/=. 

alata,    19lf. 

height  of  species  and  crosses, 

Langsdorfjli,   191 

longifiora,  69 

paniculata,   192 

rustica,   192 

tahacum,    192 
Nordic  factors,  250 
Normans,  260 
Norsemen,  258 
Nucleus,   36 

Oats,  114 


148, 


149 


Oogenesis,  37 
Open  door,  265 
Ostrich,  210 

Papaver,    144 

Paramecium,    202 

Parasitism,  22 

Parthenogenesis,   206 

Pasteur,  235,  237 

Payne,  235 

Pearl,  80/.,  83^.,  87 

Pearson,  92 

Peas,    148 

Pellew,   170,   172 

Pentstemon,   144 

Peredinece,   27 

Petunia,   117,    144 

Phaseolus,   139 

Phillips,  159 

Pimm,  139 

Pcellman,  238 

Pollen  grains,   formation  of  in  the 

lily,  40 
Polydactyly,  230 
Polypodiacece,  32 
Preston,  235 
Primula,   144 
Protandry,    23,    135,    201 
Protogyny,  23,  201 
Protozoa,  21 
Pumpkin,    221 
Punnett,  159 

Quantitative  characters,  inheritance 
of,    60^. 

Races,  intermingling  of  and  national 

stamina,  245^. 
Racial  types,  250 
Radish,  192 
RaivwnculacecB,  32 
Raphanus   sativus,    192 
Rat,    188 

curves    showing   body    weight   of, 
107/. 

inbreeding       experiments       with, 
101^.,    105 


284 


INDEX 


Rat,  size  of  litters  of,  109 

Eeduction   of  heterozygous   individ- 
uals and  allelomorphic  pairs,  90 

Remick,  92 

Reproduction,    among    animals    and 
plants,  20 
asexual,    17,   21,   22,    30,   78 
sexual,   17,  20^.,  26,  201,  205 
sexual,  origin  of,  2()ff. 

Retina,  pigmentary  degeneration  of, 
231 

Rhopalura,    26 

Rice,  114 

Ripley,  257 

Ritzema-Bos,    101^.,    188 

Roberts,  153 

Robbins,  92 

Roman  Empire,  259 

Romans,  259 

Rome,   251 

Rommel,  110 

Rosacece,  32 

Rotifer,   158,   170,  202 

SaccuUna,  23 

Sageret,  141,  144,  15G,  192 

Sax,   157 

Saxons,  258,  2G0 

Scandinavians,  257 

Schizopliytes,   26 

School  for  mothers,  227 

Scotch,  259f. 

Scotland,  257,  260 

Sex,  determination  of,  43^. 

Sexual  dimorphism,  26 

Self-fertilization  as  means  of  obtain- 
ing homozygosity,  97 

Self- sterility,  25,  33 

Sex-linked  characters,  47^. 
inheritance  of,  48/". 

Sex,  origin  of,  201 

Sex  ratio,  106 

Shamel,  119 

Sheep,  210^. 

Shull,  A.  F.,  112/=.,  158,  169 

Shull,  G.  IL,  118f.,  168/. 


Silkworms,   158 
Spain,   257/. 
Spaniard,  262 
Spermatogenesis,    37 
Spermatozoon,  entrance  of,  through 

membrane  of  egg,  42 
Spliwrocarpus,   45 

Donnellii,  45 

texanus,  45 
Spirogyra,   27 
Sponges,  21^. 
Spores,   29 
Sporophyte,  29 
Squashes,   221 
Sterility,    188^. 
Stevenson,  233 
Stone  Age,  258 
Strasburger,  45 
Sturtevant,  184 
Htylonychia  pustulata,  78 
Swine,  210/.,  213 
Synapsis,  37 
Syndactyly,  230 

Talmud,  Hebraic,  14 
Tapeworm,  23/. 
Tobacco,    114,    148,   156 
corolla  length  of,  70 
Tomatoes,    114,    148,    221 
Toyama,  158 
Trochelminthes,  21/.,  202 
Tropa^olum,  144 
Tunicates,  23 
Turanian,  259;    race,  258 
Turbellarians,  23 
Tuttle,  235 

Ulothrix,  26,  28 

Unit  of  heredity,  77 

United  Kingdom,  257 

United  States,  253^.,  260,  262,  264 

Ustilago  maydis,  118 

Venable,  235 
Verhascum,    142,    144 


INDEX 


285 


Wales,  261 

Weismann,  101^.,  188,  201,  206 
Wentworth,  92,  112 
Westemiarck,  238,  243 
Wheat,  114,  197,  210 
Wheeler,  77 
Whitney,   112,   158 
Wiegmann,  141,  144,  154 
William  the  Conqueror,  260 
Woods,  233 


Wooli'v,  235 

Wrigli't,   110/-.,   100/.,   188 

Xeroderma   pigmentoaum,   231 

Yak,  192 

Yellow  mouse,  179 

Youatt,  220 

Zero,  237,  239 


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